added github action to enable auto-merge in PRs

Added two GitHub actions to enable auto-merge in the repository.

This resolves polkadot-fellows#41

## [Auto-Merge-Bot](https://github.com/paritytech/auto-merge-bot)

This bot allows _public_ members of the organization and the author of the PR to enable/disable auto-merge in a PR. It also has a field for a list of users who are also allowed to trigger the bot. For that we have the second action.

.github/workflows/auto-merge.yml

@@ -0,0 +1,18 @@
+name: Auto Merge Bot
+
+on:
+  # GitHub considers PRs as issues
+  issue_comment:
+    types: [created]
+
+jobs:
+  set-auto-merge:
+    runs-on: ubuntu-latest
+    # Important! This forces the job to run only on comments on Pull Requests that starts with '/merge'
+    if: ${{ github.event.issue.pull_request && startsWith(github.event.comment.body, '/merge') }}
+    steps:
+      - name: Set auto merge
+        uses: paritytech/[email protected]
+        with:
+          GITHUB_TOKEN: '${{ github.token }}'
+          MERGE_METHOD: "SQUASH"