ci: define permissions for contributing comment workflow

Explicitely stating required permissions is considered best practice.
This case was detected by Poutine, see
https://github.com/boostsecurityio/poutine/blob/main/docs/content/en/rules/default_permissions_on_risky_events.md.

.github/workflows/contributing.yml

@@ -7,6 +7,10 @@ on:
     types:
       - opened
 
+permissions:
+  contents: read # for reading contributing file
+  issues: write # for posting PR comment
+
 jobs:
   comment:
     runs-on: ubuntu-latest