Update .github/workflows/pkgdown.yaml

Co-authored-by: brookslogan <[email protected]>

Author: dshemetov

.github/workflows/pkgdown.yaml

@@ -16,6 +16,7 @@ name: pkgdown
 
 jobs:
   pkgdown:
+    # only build docs on the main repository and not forks
     if: github.repository_owner == 'cmu-delphi'
     runs-on: ubuntu-latest
     # Only restrict concurrency for non-PR jobs