Create pr-check.yml

For users with many open PRs, creates a warning message in the PR description when opening a new PR, encouraging users to complete existing PRs before opening new ones.

.github/workflows/pr-check.yml

@@ -0,0 +1,26 @@
+name: PR Check
+
+on:
+  pull_request:
+    types:
+      - opened
+
+jobs:
+  check-pr:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v2
+
+      - name: Get PR count
+        id: pr-count
+        run: |
+          PR_COUNT=$(curl -s -H "Authorization: Bearer ${{ secrets.GITHUB_TOKEN }}" \
+          "https://api.github.com/repos/naturalcrit/homebrewery/pulls? state=open&head=${{ github.actor }}:${{ github.head_ref }}" | jq '. | length')
+          echo "::set-output name=pr_count::$PR_COUNT"
+
+      - name: Update PR description
+        if: ${{ steps.pr_count.outputs.pr_count }} -ge 1
+        run: |
+          gh pr edit ${{ github.event.number }} --body "You already have ${{ steps.pr-count.outputs.pr_count }} PRs open. Consider completing some of your existing PRs before opening new ones."