Add CONTRIBUTING.md

Author: threema-danilo

Diff for: CONTRIBUTING.md

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+# Contributing to Threema Web
+
+Contributions to Threema Web are welcome! Please follow these steps:
+
+## 1. Claim an Issue
+
+To avoid having multiple people working on the same problem, make sure that an
+issue exists for it. If one already exists, add a comment that you would like
+to work on that problem. If you have questions, let us know, we'll try to help
+you with the implementation.
+
+## 2. Code
+
+Do the necessary changes. After you're done, please run our tests and linting
+checks:
+
+    npm run build
+    chromium tests/testsuite.html
+    npm run lint
+
+If no errors appear, you're good to go.
+
+## 3. Create a Pull Request
+
+Please create a Github pull request against [our
+Repository](https://github.com/threema-ch/threema-web). Make sure to reference
+the issue in the description.
+
+## 4. Sign the CLA
+
+We require all contributors to sign a CLA. You can find it here:
+
+https://cla-assistant.io/threema-ch/threema-web
+
+Please sign the CLA with your Github account so that we can accept your
+pull request.