Add README

Author: dentarg

README.md

@@ -0,0 +1,24 @@
+# Gem Compare
+
+For pull requests with a title matching `/Bump (.+) from (.+) to (.+)/`, it posts a comment with the output from `gem compare` and `gem compare --diff`. Uses https://rubygems.org/gems/gem-compare (https://github.com/fedora-ruby/gem-compare).
+
+To use in your repo, add a workflow like this:
+
+````yaml
+name: Gem Compare
+
+on:
+  pull_request:
+    types:
+      - opened
+      - reopened
+
+jobs:
+  compare:
+    if: startsWith(github.head_ref, 'dependabot/bundler/')
+    runs-on: ubuntu-latest
+    steps:
+      - uses: dentarg/gem-compare@v1
+        with:
+          github-token: ${{ secrets.GITHUB_TOKEN }}
+````