add some documentation

Author: dey4ss

Diff for: .github/workflows/ci.yml

@@ -55,6 +55,10 @@ jobs:
         if: matrix.name == 'gcc-6'
         # Recent versions of Github's checkout action do not run on older Ubuntu versions because they use a too recent
         # Node.js version. Thus, we have to checkout the code manually.
+        # Doing so is a bit tricky when it comes to PRs from forks (see #249 for details). The general idea here is that
+        # we access Github's context information for the event triggering the action's execution and use some details on
+        # the PR's HEAD if given. Otherwise (for executions due to master updates), we still use the provided
+        # environment variables.
         run: |
           apt-get update
           apt-get install -y git