Add CONTRIBUTING.md

Taken from boots and added info for building the protobuf generated
files.

CONTRIBUTING.md

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+## Hello Contributors!
+
+Thx for your interest!
+We're so glad you're here.
+
+### Important Resources
+
+#### bugs: [https://github.com/tinkerbell/tink/issues](https://github.com/tinkerbell/tink/issues)
+
+### Code of Conduct
+
+Available via [https://github.com/tinkerbell/tink/blob/master/.github/CODE_OF_CONDUCT.md](https://github.com/tinkerbell/tink/blob/master/.github/CODE_OF_CONDUCT.md)
+
+### Environment Details
+
+[https://github.com/tinkerbell/tink/blob/master/Makefile](https://github.com/tinkerbell/tink/blob/master/Makefile)
+
+### How to Submit Change Requests
+
+Please submit change requests and / or features via [Issues](https://github.com/tinkerbell/tink/issues).
+There's no guarantee it'll be changed, but you never know until you try.
+We'll try to add comments as soon as possible, though.
+
+### How to Report a Bug
+
+Bugs are problems in code, in the functionality of an application or in its UI design; you can submit them through [Issues](https://github.com/tinkerbell/tink/issues).
+
+## Code Style Guides
+
+#### Protobuf
+
+Please ensure protobuf related files are generated along with _any_ change to a protobuf file.
+CI will enforce this, but its best to commit the generated files along with the protobuf changes in the same commit.
+Handling of protobuf deps and generating the go files are both handled by the [protoc.sh](./protos/protoc.sh) script.
+Both go & protoc are required by protoc.sh, these are both installed and used if using nix-shell.