Link to CI script in compiling documentation

Because this helps document the way CBMC is currently built in CI. This
is usefull both for reference compared to local builds and for working
out why CI builds are failing on PRs.

Author: thomasspriggs

COMPILING.md

@@ -8,7 +8,11 @@ following environments:
 - Microsoft Visual Studio 2019
 
 The above environments are currently tested as part of our continuous
-integration system. The environments below have been used successfully in the
+integration system. It separately tests both the CMake build system and the
+hand-written make files. The latest build steps being used in CI can be
+[referenced here](https://github.com/diffblue/cbmc/blob/develop/.github/workflows/pull-request-checks.yaml).
+
+The environments below have been used successfully in the
 past, but are not actively tested:
 
 - Solaris 11