Extract development documentation from README

[endorama]

Part of #372

This PR extract the part related to building fulcio for development from README to it's own file in docs/ folder.

/cc @nsmith5

[nsmith5]

I like that this isn't in the README and more but it definitely needs so more work. Could we a link to the global sigstore contributor docs here https://github.com/sigstore/.github/blob/main/CONTRIBUTING.md to help folks get an idea of what contributing looks like.

[endorama]

I checked that and when creating a new issue the global CONTRIBUTING file is provided as guide.
I actually thought of moving it to a CONTRIBUTING.md in the repo, but didn't want to override the global one.

I think having the global one suggested by GitHub and a specific developer docs is a decent trade off. It would make sense to link from this file to the global contributing policy, I'll add it.