[SPARK-51485] Add How to use in your apps section to README.md

### What changes were proposed in this pull request?

This PR aims to add `How to use in your apps` section to `README.md`.

### Why are the changes needed?

To illustrate how to use this `Swift` library in the Swift applications.

### Does this PR introduce _any_ user-facing change?

No, this is a documentation change.

### How was this patch tested?

Manual review.

### Was this patch authored or co-authored using generative AI tooling?

No.

Closes #11 from dongjoon-hyun/SPARK-51485.

Authored-by: Dongjoon Hyun <[email protected]>
Signed-off-by: Dongjoon Hyun <[email protected]>

Author: dongjoon-hyun

README.md

@@ -13,3 +13,82 @@ So far, this library project is tracking the upstream changes like the [Apache S
 - [gRPC Swift Protobuf 1.0 (March 2025)](https://github.com/grpc/grpc-swift-protobuf/releases/tag/1.1.0)
 - [gRPC Swift NIO Transport 1.0 (March 2025)](https://github.com/grpc/grpc-swift-nio-transport/releases/tag/1.0.1)
 - [Apache Arrow Swift](https://github.com/apache/arrow/tree/main/swift)
+
+## How to use in your apps
+
+Create a Swift project.
+```
+$ mkdir SparkConnectSwiftApp
+$ cd SparkConnectSwiftApp
+$ swift package init --name SparkConnectSwiftApp --type executable
+```
+
+Add `SparkConnect` package to the dependency like the following
+```
+$ cat Package.swift
+import PackageDescription
+
+let package = Package(
+  name: "SparkConnectSwiftApp",
+  platforms: [
+    .macOS(.v15)
+  ],
+  dependencies: [
+    .package(url: "https://github.com/apache/spark-connect-swift.git", branch: "main")
+  ],
+  targets: [
+    .executableTarget(
+      name: "SparkConnectSwiftApp",
+      dependencies: [.product(name: "SparkConnect", package: "spark-connect-swift")]
+    )
+  ]
+)
+```
+
+Use `SparkSession` of `SparkConnect` module in Swift.
+
+```
+$ cat Sources/main.swift
+
+import SparkConnect
+
+let spark = try await SparkSession.builder.getOrCreate()
+print("Connected to Apache Spark \(await spark.version) Server")
+
+let statements = [
+  "DROP TABLE IF EXISTS t",
+  "CREATE TABLE IF NOT EXISTS t(a INT)",
+  "INSERT INTO t VALUES (1), (2), (3)",
+]
+
+for s in statements {
+  print("EXECUTE: \(s)")
+  _ = try await spark.sql(s).count()
+}
+print("SELECT * FROM t")
+try await spark.sql("SELECT * FROM t").show()
+
+await spark.stop()
+```
+
+Run your Swift application.
+
+```
+$ swift run
+...
+Connected to Apache Spark 4.0.0 Server
+EXECUTE: DROP TABLE IF EXISTS t
+EXECUTE: CREATE TABLE IF NOT EXISTS t(a INT)
+EXECUTE: INSERT INTO t VALUES (1), (2), (3)
+SELECT * FROM t
++---+
+| a |
++---+
+| 2 |
+| 1 |
+| 3 |
++---+
+```
+
+You can find this example in the following repository.
+- https://github.com/dongjoon-hyun/spark-connect-swift-app