docs: compatibility table for CLI/library versions

Describe the compatibility model, and add a table linking
the CLI and library versions.

Author: rix0rrr

COMPATIBILITY.md

@@ -0,0 +1,93 @@
+# CDK CLI and AWS Construct Library compatibility table
+
+The table below show what version of the CDK CLI is required for a particular
+version of the AWS Construct Library:
+
+The table below contains the specific versions of the schema, along with the
+CLI that supports them and the Construct Library that uses them.
+
+| Library version | Minimum CLI Version | Schema Version |
+|-----------------|---------------------|----------------|
+| 2.174.0         | 2.174.0             |  39            |
+| 2.182.0         | 2.1000.0            |  40            |
+| (unreleased)    | 2.1003.0            |  41            |
+
+How to use this table: find your library version in the leftmost column.
+If it's not in there, find the highest version lower than your library version.
+The middle column indicates the minimum version of the CDK CLI you need to use.
+
+## How it works
+
+(This section describes how the protocol between App and CLI is versioned and
+is purely for reference; the table above contains the most important information.)
+
+The compatibility between the construct library and the CLI is based on the
+version of the [Cloud Assembly Schema](./packages/@aws-cdk/cloud-assembly-schema)
+used by the construct library and supported by the CLI app. The app writes
+a particular version `X` of the Cloud Assembly, and the CLI can read all versions
+up to and including the version `Y` that was current when it was released. We
+need `X <= Y`.
+
+```
+┌────────┐          ┌─────────┐           ┌──────────┐
+│        │ (writes) │         │  (reads)  │          │
+│  App   │─────────▶│ cdk.out │◀──────────│   CLI    │
+│        │          │         │           │          │
+└────────┘  schema  └─────────┘  schemas  └──────────┘
+             vX                  <= vY
+```
+
+### Compatibility rule for CLI < 2.1000.0
+
+Before CDK CLI `2.1000.0`, the CLI and the library were released in lockstep
+with the same version number, and so the versions `X` and `Y` above are the
+same for a given version number.
+
+An easy compatibility rule therefore is: the CLI supports apps written against
+the AWS Construct Library with the same version or lower. It might support more
+versions as well, but it *at least* supports those.
+
+This works because `v(App) <= v(CLI)` guarantees that `X <= Y`.
+
+### Compatibility rule for CLI >= 2.1000.0
+
+Since CDK CLI `2.1000.0`, the CLI and construct library have two different
+versions. The only rule that relates `X` and `Y` involves the *release date* of
+the respective packages. The CLI version that supports a particular schema version
+is always released before a CDK app starts emitting cloud assemblies using
+that schema.
+
+A general rule therefore is: a particular library version's Cloud Assemblies
+are supported by the CLI that is current at the time of the library's release,
+as well as every newer CLI version. It might be supported by older versions, but
+it is *at least* supported by those.
+
+This works because the CLI that is current at time of release of a
+particular library version can always handle the output of that library
+version and every version before it.
+
+In a visual example:
+
+```
+                             this CLI version...
+                                    ⋎
+                                    o
+CLI           │     │     │         │       │      │     │      │
+        ─────┬┴─────┴┬────┴──┬────┬─┴────┬──┴──────┴┬────┴───┬──┴───→ (time)
+Library      │       │       │    │      │          │        │
+             ⋏       ⋏       ⋏    ⋏      ⋏
+   ...supports at least these library versions
+```
+
+And the other way around:
+
+```
+                             this library version...
+                                         ⋎
+                                         o
+Library      │       │       │    │      │          │        │
+        ─────┴┬─────┬┴────┬──┴────┴─┬────┴──┬──────┬┴────┬───┴──┬────→ (time)
+CLI           │     │     │         │       │      │     │      │
+                                    ⋏       ⋏      ⋏     ⋏      ⋏
+                                ...is supported by these CLI versions
+```

README.md

@@ -62,3 +62,12 @@ Here are the packages in this repository. See the README of each package for mor
 Every package comes with its own unit tests. There is a companion repository to this one containing the integration tests. You can find it here: <https://github.com/aws/aws-cdk-cli-testing>
 
 See the [contributing guide](./CONTRIBUTING.md) for more information on this repository.
+
+## Compatibility
+
+The CLI is always able to process CDK apps using a version of the Construct Library
+released before it. We recommend you always keep the CLI version up-to-date to get the
+latest features.
+
+For more details on the compatibility model and specific versions that work together,
+see [COMPATIBILITY.md](COMPATIBILITY.md).