Elaborate what "test compiler performance" actually does.

Author: moiseev

docs/ContinuousIntegration.md

@@ -150,11 +150,19 @@ preset=buildbot_incremental,tools=RA,stdlib=RD,smoketest=macosx,single-thread
 
 ### Testing Compiler Performance
 
+The following 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
+
 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
 
+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