Adjust license to include Apache2 and add contribution guidelines similar to Rust-Analyzer

[Byron]

Right now is not a good time to contribute as a few parts are missing:

• I still want to have a dual license like many popular Rust projects do, also offering Apache 2
• There should be well thought-out contribution guidelines that learn from the best, I heard Rust Analyzer is a great example

[Byron]

@joshtriplett Would you be OK to adjust your contribution to include the Apache 2 license so this project can go dual-licensed? Thanks for your consideration.

[joshtriplett]

Yes, of course. 👍

[Byron]

We have a contribution guide and the project is dual licensed as well. It seems OK to close this one or bring up new issues otherwise.