Merge pull request swiftlang#12363 from apple/ci-doc-compiler-performance

[doc] Document "please test compiler performance"

Author: moiseev

docs/ContinuousIntegration.md

@@ -148,6 +148,22 @@ preset=buildbot_incremental,tools=RA,stdlib=RD,smoketest=macosx,single-thread
 
 ```
 
+### Testing Compiler Performance
+
+Platform        | Comment | Check Status
+------------    | ------- | ------------
+macOS platform  | @swift-ci Please test compiler performance       | Compiles full source compatibility test suite and measures compiler performance
+macOS platform  | @swift-ci Please smoke test compiler performance | Compiles a subset of source compatibility test suite and measures compiler performance
+
+These commands will:
+
+1. Build a set of projects from the compatibility test suite
+2. Collect counters and timers reported by the compiler
+3. Compare the obtained data to the baseline (stored in git) and HEAD (version of a compiler built without the PR changes)
+4. Report the results in a pull request comment
+
+For the detailed explanation of how compiler performance is measured, please refer to [this document](https://github.com/apple/swift/blob/master/docs/CompilerPerformance.md).
+
 ## Cross Repository Testing
 
 Simply provide the URL from corresponding pull requests in the same comment as "@swift-ci Please test" phrase. List all of the pull requests and then provide the specific test phrase you would like to trigger. Currently, it will only merge the main pull request you requested testing from as opposed to all of the PR's.