ci macos: explain GITHUB_REF parsing

.github/workflows/main.yml

@@ -35,6 +35,7 @@ jobs:
         RAW_THERAPEE_MINOR: '7'
         C_FLAGS: -Xpreprocessor -fopenmp /usr/local/lib/libomp.dylib -I/usr/local/include -I/usr/local/opt/gdk-pixbuf/include -I/usr/local/opt/libiconv/include -I/usr/local/opt/libxml2/include -I/usr/local/opt/expat/include -I/usr/local/opt/llvm/include
       run: |
+        # GITHUB_REF is the ref that triggered the build, like refs/heads/new-feature - the next line parses that to REF: the branch name only (new-feature)
         REF=${GITHUB_REF##*/}
         mkdir build && cd build
         cmake \