Document specially integrated modules and integration process.

Author: rxwei

README.md

@@ -9,3 +9,61 @@ 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`                                                    |
+
+### Branching scheme
+
+#### Development branch
+
+The `main` branch is the branch for day-to-day development. Generally, you should create PRs against this branch.
+
+#### Swift integration branches
+
+Branches whose name starts with `swift/` are Swift integration branches similar to those in [apple/llvm-project](https://github.com/apple/llvm-project). For each branch, dropping the `swift/` prefix is the corresponding branch in [apple/swift](https://github.com/apple/swift).
+
+| apple/swift branch  | apple/swift-experimental-string-processing branch     |
+| ------------------- | ----------------------------------------------------- |
+| main                | swift/main                                            |
+| release/5.7         | swift/release/5.7                                     |
+| ...                 | swift/...                                             |
+
+A pair of corresponding branches are expected to build successfully together and pass all tests.
+
+### Integration workflow
+
+To integrate the latest changes in apple/swift-experimental-string-processing to apple/swift, carefully follow the workflow: 
+
+- Create a pull request in apple/swift-experimental-string-processing from `main` to `swift/main`, e.g. "[Integration] main -> swift/main".
+- Create a pull request in apple/swift.
+  - If apple/swift needs to be modified to work with the latest `main` in apple/swift-experimental-string-processing, create a pull request that contains those modifications.
+  - Otherwise, create a dummy pull request by, for example, editing a character in README. This is so that we can trigger CI to test Swift against the latest changes in apple/swift-experimental-string-processing.
+- In the apple/swift pull request, 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
+  ```
+- When all tests pass, attach the successful test results to the pull request in apple/swift-experimental-string-processing.
+- When approved:
+  - Merge the pull request in apple/swift-experimental-string-processing.
+  - If the pull request in apple/swift is not a dummy, merge it.
+
+### 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.