Include a CONTRIBUTING.md with guidelines for collaborators

[Sollimann]

Add guidelines for collaborators

A few ones

• Only add new node primitive if target behavior cannot be created with existing primitives and there is actual demand / need for this behavior
• Keep the crate lean with as few dependencies as possible. This keeps binary sizes small, makes the library relevant for embedded applications, the library can compile to WASM (does not work with io libraries like tokio), less chance of downstream breaking changes etc..