Document use of the build plugin (#2176)

### Motivation:

To ease use of the `grpc-swift-protobuf` Swift Package Manager build
plugin.

### Modifications:

Documentation added.

### Result:

Greater understanding?

---------

Co-authored-by: George Barnett <[email protected]>

Author: rnro

Sources/GRPCCore/Documentation.docc/Articles/Generating-stubs.md

@@ -4,25 +4,109 @@ Learn how to generate stubs for gRPC Swift from a service defined using the Prot
 
 ## Overview
 
-### Using protoc
-
 If you've used Protocol Buffers before then generating gRPC Swift stubs should be simple. If you're
 unfamiliar with Protocol Buffers then you should get comfortable with the concepts before
 continuing; the [Protocol Buffers website](https://protobuf.dev/) is a great place to start.
 
-The [`grpc-swift-protobuf`](https://github.com/grpc/grpc-swift-protobuf) package provides
-`protoc-gen-grpc-swift`, a program which is a plugin for the Protocol Buffers compiler, `protoc`.
+You can use the `protoc` plugin from the command line directly, or you can make use of a 
+ [Swift Package Manager build plugin](https://github.com/swiftlang/swift-package-manager/blob/main/Documentation/Plugins.md) 
+convenience which adds the stub generation to the Swift build graph.
+You may use the build plugin either from the command line or from Xcode.
+
+## Using the build plugin
+
+The build plugin (`GRPCProtobufGenerator`) is a great choice for convenient dynamic code generation, however it does come with some limitations.
+Because it generates the gRPC Swift stubs as part of the build it has the requirement that `protoc` must be available 
+at compile time. This requirement means it is not a good fit for library authors who do not have
+direct control over this.
+
+The build plugin detects `.proto` files in the source tree and invokes `protoc` once for each file 
+(caching results and performing the generation as necessary).
+
+### Adoption
+You must adopt Swift Package Manager build plugins on a per-target basis by modifying your package manifest 
+(`Package.swift` file). To do this, declare the grpc-swift-protobuf package as a dependency and add the plugin 
+to your desired targets.
+
+For example, to make use of the plugin for generating gRPC Swift stubs as part of the 
+`echo-server` target:
+```swift
+targets: [
+   .executableTarget(
+     name: "echo-server",
+     dependencies: [
+       // ...
+     ],
+     plugins: [
+       .plugin(name: "GRPCProtobufGenerator", package: "grpc-swift-protobuf")
+     ]
+   )
+ ]
+```
+Once this is done you need to ensure that the `.proto` files to be used for generation 
+are included in the target's source directory and that you have defined at least one configuration file.
+
+### Configuration
+
+You must provide a configuration file in the directory which encloses all `.proto` files (in the same directory or a parent). 
+Configuration files, written in JSON, tell the build plugin about the options used for `protoc` invocations. 
+You must name the file `grpc-swift-proto-generator-config.json` and structure it in the following format:
+```json
+{
+  "generate": {
+    "clients": true,
+    "servers": true,
+    "messages": true,
+  },
+  "generatedSource": {
+    "accessLevelOnImports": false,
+    "accessLevel": "internal",
+  }
+  "protoc": {
+    "executablePath": "/opt/homebrew/bin/protoc"
+    "importPaths": [
+      "../directory_1",
+    ],
+  },
+}
+```
 
-> `protoc-gen-grpc-swift` only generates gRPC stubs, it doesn't generate messages. You must use
-> `protoc-gen-swift` to generate messages in addition to gRPC Stubs.
+The options do not need to be specified and each have default values.
+
+| Name                                   | Possible Values                            | Default      | Description                                         |
+|----------------------------------------|--------------------------------------------|--------------|-----------------------------------------------------|
+| `generate.servers`                     | `true`, `false`                            | `true`       | Generate server stubs                               |
+| `generate.clients`                     | `true`, `false`                            | `true`       | Generate client stubs                               |
+| `generate.messages`                    | `true`, `false`                            | `true`       | Generate message stubs                              |
+| `generatedSource.accessLevelOnImports` | `true`, `false`                            | `false`      | Whether imports should have explicit access levels  |
+| `generatedSource.accessLevel`          | `"public"`, `"package"`, `"internal"`      | `"internal"` | Access level for generated stubs                    |
+| `protoc.executablePath`                | N/A                                        | `null`†      | Path to the `protoc` executable                     |
+| `protoc.importPaths`                   | N/A                                        | `null`‡      | Import paths passed to `protoc`                     |
+
+† The Swift Package Manager build plugin infrastructure will attempt to discover the executable's location if you don't provide one.
 
-To generate gRPC stubs for your `.proto` files you must run the `protoc` command with
+‡ If you don't provide any import paths then the path to the configuration file will be used on a per-source-file basis. 
+
+Many of these options map to `protoc-gen-grpc-swift` and `protoc-gen-swift` options.
+
+If you require greater flexibility you may specify more than one configuration file.
+Configuration files apply to all `.proto` files equal to or below it in the file hierarchy. A configuration file
+lower in the file hierarchy supersedes one above it.
+
+### Using protoc
+
+The [`grpc-swift-protobuf`](https://github.com/grpc/grpc-swift-protobuf) package provides
+`protoc-gen-grpc-swift`, a program which is a plugin for the Protocol Buffers compiler, `protoc`.
+To generate gRPC stubs for your `.proto` files directly you must run the `protoc` command with
 the `--grpc-swift_out=<DIRECTORY>` option:
 
 ```console
 protoc --grpc-swift_out=. my-service.proto
 ```
 
+> `protoc-gen-grpc-swift` only generates gRPC stubs, it doesn't generate messages. You must use
+> `protoc-gen-swift` to generate messages in addition to gRPC Stubs.
+
 The presence of `--grpc-swift_out` tells `protoc` to use the `protoc-gen-grpc-swift` plugin. By
 default it'll look for the plugin in your `PATH`. You can also specify the path to the plugin
 explicitly:
@@ -69,7 +153,7 @@ allows you to specify a mapping from `.proto` files to the Swift module they are
 allows the code generator to add appropriate imports to your generated stubs. This is described in
 more detail in the [SwiftProtobuf documentation](https://github.com/apple/swift-protobuf/blob/main/Documentation/PLUGIN.md).
 
-#### Building the plugin
+#### Building the protoc plugin
 
 > The version of `protoc-gen-grpc-swift` you use mustn't be newer than the version of
 > the `grpc-swift-protobuf` you're using.