Document specially integrated modules and integration process.

Author: rxwei

README.md

@@ -9,3 +9,49 @@ See [Declarative String Processing Overview][decl-string]
 ## Requirements
 
 - [Swift Trunk Development Snapshot](https://www.swift.org/download/#snapshots) DEVELOPMENT-SNAPSHOT-2022-02-03 or later.
+
+## Integration with Swift
+
+`_MatchingEngine`, `_CUnicode` and `_StringProcessing` are specially integrated modules that are built as part of apple/swift.
+
+Specifically, `_MatchingEngine` contains the parser for regular expression literals and is built both as part of the compiler and as a core library. `_CUnicode` and `_StringProcessing` are built together as a core library named `_StringProcessing`.
+
+| Module              | Swift toolchain component                                                            |
+| ------------------- | ------------------------------------------------------------------------------------ |
+| `_MatchingEngine`   | `SwiftCompilerSources/Sources/ExperimentalRegex` and `stdlib/public/_MatchingEngine` |
+| `_CUnicode`         | `stdlib/public/_StringProcessing`                                                    |
+| `_StringProcessing` | `stdlib/public/_StringProcessing`                                                    |
+
+## Pull request testing
+
+Pull requests are tested via Swift CI.  Unlike other packages, sources from this package (the `_MatchingEngine` module) are used directly by the Swift compiler, so any change to these sources will be cross-tested with the compiler using `build-script` in addition to SwiftPM testing. Other sources are tested using SwiftPM only.
+
+### Workflow
+
+- Create pull requests.
+  - Create a pull request in apple/swift-experimental-string-processing from `main` to `swift/main`, e.g. "[Integration] main -> swift/main".
+  - If apple/swift needs to be modified together, create a pull request in apple/swift.
+- Trigger CI.
+  - In the apple/swift-experimental-string-processing pull request, trigger CI using the following command (replacing `<PR NUMBER>` with the apple/swift pull request number, if any):
+    ```
+    apple/swift#<PR NUMBER> # use this line only if there is an corresponding apple/swift PR
+    @swift-ci please test
+    ```
+  - In the apple/swift pull request (if any), trigger CI using the following command (replacing `<PR NUMBER>` with the apple/swift-experimental-string-processing pull request number):
+    ```
+    apple/swift-experimental-string-processing#<PR NUMBER>
+    @swift-ci please test
+    ```
+- Merge when approved.
+  - Merge the pull request in apple/swift-experimental-string-processing as a **merge commit**.
+  - Merge the pull request in apple/swift, if any.
+
+## Development notes
+
+Compiler integration can be tricky. Use special caution when developing `_MatchingEngine`, `_CUnicode` and `_StringProcessing` modules.
+
+- Do not change the names of these modules without due approval from compiler and infrastructure teams.
+- Do not modify the existing ABI between the regular expression parser and the Swift compiler unless absolutely necessary. 
+- Always minimize the number of lockstep integrations, i.e. when apple/swift-experimental-string-processing and apple/swift have to change together.
+- In `_StringProcessing`, do not write fully qualified references to symbols in `_CUnicode`, and always wrap `import _CUnicode` in a `#if canImport(_CUnicode)`. This is because `_CUnicode` is built as part of `_StringProcessing` with CMake.
+- In `_MatchingEngine`, do not write fully qualified references to `_MatchingEngine` itself. This is because `_MatchingEngine` is built as `ExperimentalRegex` in `SwiftCompilerSources/` with CMake.