chore: update PR template and add info for new contribs

.github/PULL_REQUEST_TEMPLATE.md

@@ -7,10 +7,7 @@
 
 <!-- TODO: Verify the following, checking each box with an 'x' between the brackets: [x] -->
 
-- [ ] I branched from develop
-- [ ] My pull request is against develop
-- [ ] My code follows the [style guide](https://developers.google.com/blockly/guides/modify/web/style-guide)
-- [ ] I ran `npm run format` and `npm run lint`
+- [ ] I [validated my changes](https://developers.google.com/blockly/guides/contribute/core#making_and_verifying_a_change)
 
 ## The details
 ### Resolves

.github/workflows/welcome_new_contributors.yml

@@ -0,0 +1,37 @@
+on:
+  pull_request_target:
+    types:
+      - opened
+name: Welcome new contributors
+jobs:
+  welcome:
+    runs-on: ubuntu-latest
+    permissions:
+      pull-requests: write
+    steps:
+      - uses: actions/first-interaction@v1
+        with:
+          repo-token: ${{ secrets.GITHUB_TOKEN }}
+          pr-message: >
+            Welcome! It looks like this is your first pull request in Blockly,
+            so here are a couple of tips:
+
+              - You can find tips about contributing to Blockly and how to
+              validate your changes on our
+              [developer site](https://developers.google.com/blockly/guides/contribute/core#making_and_verifying_a_change).
+
+              - All contributors must sign the Google Contributor License
+              Agreement (CLA). If the google-cla bot leaves a comment on this
+              PR, make sure you follow the instructions.
+
+              - We use [conventional commits](https://www.conventionalcommits.org/)
+              to make versioning the package easier. Make sure your commit
+              message is in the [proper format](https://developers.google.com/blockly/guides/contribute/get-started/commits)
+              or [learn how to fix it](https://developers.google.com/blockly/guides/contribute/get-started/commits#fixing_non-conventional_commits).
+
+              - If any of the other checks on this PR fail, you can click on
+              them to learn why. It might be that your change caused a test
+              failure, or that you need to double-check the
+              [style guide](https://developers.google.com/blockly/guides/contribute/core/style_guide).
+
+            Thank you for opening this PR! A member of the Blockly team will review it soon.