doc: clarify practices for submitting updated patch versions
The `SubmittingPatches` documentation briefly mentions that related
patches should be grouped together in their own e-mail thread. Expand on
this to explicitly state that updated versions of a patch series should
also follow this. Also provide add a link to existing documentation from
`MyFirstContribution` that provides detailed instructions on how to do
this via `git-send-email(1)`.
Signed-off-by: Justin Tobler <jltobler@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>