Improve the 'first_contribution_swift' by adding instructions for how to
rebuild swift following a rebase.
Change-Id: If5c91dc4e1e8d1712bbd8b326c675967fb4b8c15
In this change, there are proposed improvement to Swift documentation aimed at
first-time contributors. They include a simplification of the Getting
Started page and a new page with some basic instructions/commands that the
first-time contributor should know. In addition, it shows some common errors
that the first-time contributor may find when executing git rebase and
information on how to track your changes.
Change-Id: I704202955093736b2f3b4102a649690a0392c6b0
