Clarify working with sources
Currently the following things are confusing:
- https://trac.torproject.org/projects/tor/browser differs from https://gitweb.torproject.org/. Explain the difference between both in a statement on each URL and/or link to instructions on working with sources on both.
- include a link to instructions on working with sources on the homepage https://trac.torproject.org/projects/tor
- There's no way to checkout sources from https://trac.torproject.org/projects/tor/browser.