Github action to require 2 reviewers (#884)

.github/label-requires-reviews.yml

+---
+  - label: 2-reviewers
+    reviews: 2

.github/labeler.yml

 ---
+2-reviewers:
+  - '.github/**/*'
+  - 'l2geth/**/*'
+  - 'ops/**/*'
+  - 'packages/batch-submitter/**/*'
+  - 'packages/contracts/**/*'
+  - 'packages/data-transport-layer/**/*'
+  - 'packages/message-relayer/**/*'
+  - 'patches/**/*'
+
 A-ci:
   - any: ['.github/**/*']
 
@@ -35,3 +45,4 @@ M-ops:
 
 M-smock:
   - any: ['packages/smock/**/*']
+

.github/workflows/label-requires-reviews.yml

+name: Label Reviews
+on:
+  pull_request_review:
+
+jobs:
+  require-reviewers:
+    runs-on: ubuntu-18.04
+    steps:
+      - uses: actions/checkout@v2
+        with:
+          ref: develop
+
+      - name: Require-reviewers
+        uses: travelperk/label-requires-reviews-action@v0.1
+        env:
+          GITHUB_TOKEN: ${{ secrets.PERSONAL_ACCESS_TOKEN}}
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