Created How to contribute (markdown)

docs/content/how-to-contribute.md

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+Contribution is like any other GitHub project:
+
+- Fork
+- Improve
+- Add integration tests in `test/tests.bats`
+- Build image and run tests using `make`
+- Document your improvements
+- Commit, push and make a pull-request
+
+#### Project architecture
+
+    ├── config                    # User: personal configurations
+    ├── target                    # Developer: default server configurations
+    └── test                      # Developer: integration tests