Create a CONTRIBUTE.md introduction

[decryphe]

It would be helpful for anyone who wants to contribute if there's a quick intro into what to consider when working on rasn.

Especially a little checklist that mentions what should be done to run manual checks, e.g. cargo test, cargo verify, cargo clippy --all-targets.

[decryphe]

Here's the further learnings from my second contribution:

• cargo test --all-targets --workspace
• Using cross to run tests for other systems, e.g. 32-bit systems - cross test --target i686-unknown-linux-gnu --workspace