Add contributors workflow and contribution guidelines (#107)

* Adds a workflow to greet first time contributors
* Update contribution guidelines

.github/workflows/welcome.yml

@@ -0,0 +1,23 @@
+name: Welcome New Contributors
+
+on:
+  issues:
+    types: [opened]
+  pull_request_target:
+    branches: [main]
+    types: [opened]
+
+jobs:
+  greet:
+    name: Greet New Contributors
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/first-interaction@v1
+        with:
+          repo-token: ${{ secrets.GITHUB_TOKEN }}
+          pr-message: |
+            Thank you for your first ever contribution to System Design 101! 🎉
+            
+            Please make sure to follow the [contribution guidelines](https://github.com/ByteByteGoHq/system-design-101/blob/main/CONTRIBUTING.md) when contributing to this project. Any PRs that don't follow the guidelines will be closed.
+
+            Thanks for choosing to contribute, and for helping make this project better! 🌟