docs: update README header, revise CONTRIBUTING tier policy, add CLA (#838)

* docs: update README header, revise CONTRIBUTING tier policy, add CLA

* ci(github): pin CLA Assistant action to commit SHA

.github/workflows/cla.yml

@@ -0,0 +1,29 @@
+name: "CLA Assistant"
+on:
+  issue_comment:
+    types: [created]
+  pull_request_target:
+    types: [opened,closed,synchronize]
+
+permissions:
+  actions: write
+  contents: write
+  pull-requests: write
+  statuses: write
+
+jobs:
+  CLAAssistant:
+    runs-on: ubuntu-latest
+    steps:
+      - name: "CLA Assistant"
+        if: (github.event.comment.body == 'recheck' || github.event.comment.body == 'I have read the CLA Document and I hereby sign the CLA') || github.event_name == 'pull_request_target'
+        uses: contributor-assistant/github-action@a895a435fcce79ecf28fbce61a4ef0f0dabc9853 # v2.3.1
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+          # The below token is optional but recommended if the CLA signatures should be stored in a private repository or you want to trigger other workflows
+          # PERSONAL_ACCESS_TOKEN: ${{ secrets.PERSONAL_ACCESS_TOKEN }}
+        with:
+          path-to-signatures: 'signatures/version1/cla.json'
+          path-to-document: 'https://github.com/AnsoCode/Sencho/blob/main/CLA.md'
+          branch: 'main'
+          allowlist: 'dependabot[bot],greenkeeper[bot]'