Revise and update the readme by EliahKagan · Pull Request #103 · gitpython-developers/gitdb

Changes worth mentioning:

- Format commands as code blocks instead of blockquotes.
  (This is particularly useful for the submodule update step, whose
  lines were inadvertently concatenated, but it also improves
  appearance overall.)

- Mention smmap as a requirement.
  (But also that it doesn't need to be separately installed.)

- Mention that gitdb-speedups is not currently maintained.

- No longer say gitdb has source code in gitorious.
  (Since that site no longer exists.)

- Call GitPython "GitPython" rather than "git-python".

- Replace the old git-python Google Groups link with a link to the
  Discussions page on the GitHub repository for GitPython.
  (This seems like the closest currently available resource.)