Add CONTRIBUTING.md

CONTRIBUTING.md

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+# How to contribute
+
+To contribute to sn0int, clone the repository and make sure both the build and
+tests pass for you:
+
+    git clone https://github.com/kpcyrd/sn0int.git
+    cd sn0int
+    # build the project
+    cargo build
+    # run regular tests
+    cargo test
+    # run tests depending on the network
+    # these might fail if a service is down
+    cargo test -- --ignored
+
+The project is loosely structured into a few folders:
+
+- `src/models/` - database models
+- `src/runtime/` - the stdlib that's exposed to lua
+- `src/engine/` - code related to lua
+- `src/sandbox/` - code related to sandboxing
+- `src/cmd/` - cli commands
+- `src/` - misc modules
+
+After you're done, make sure the build completes without any warnings and both
+tests pass successfully:
+
+    cargo test
+    cargo test -- --ignored
+
+If you want to introduce a new feature feel free to open an issue first to make
+sure your feature is a good fit for the project before implementing it.