docs: Move Developing bun from README.md to CONTRIBUTING.md

[mljlynch]

I was trying to build bun for the first time and it was confusing to me where to start.

Per #798 - I think this would be helpful to getting more people started.

A few other TODOs:

• There is not a clear list of system requirements - per the section Troubleshooting (general) at the bottom it appears that 22 GB of RAM is necessary to compile. It may be helpful to add that to a doc like this.
• It may be helpful to have an overview of how the code is structured and some description of the submodules.

[Jarred-Sumner]

Thanks for this

IMO, a single long readme designed for CMD + F is better

I think separate pages work better for websites than for markdown files on github