docs: Add how not to run on PRs to README (fixes advanced-security#17)

This fixes advanced-security#17 

It works for me like this, see https://github.com/MariaDB4j/MariaDB4j/pull/759/files

README.md

@@ -42,6 +42,7 @@ Generating and submitting a dependency snapshot using the defaults:
 
 ```
 - name: Submit Dependency Snapshot
+  if: ${{ github.event_name == 'push' }}
   uses: advanced-security/maven-dependency-submission-action@v3
 ```