MyFirstContribution: recommend the use of b4

MyFirstContribution: recommend the use of b4

The b4 tool originates from the Linux kernel community and is intended
to help mailing-list based workflows. It automates a lot of the annoying
bookkeeping tasks that contributors typically need to do: tracking the
list of recipients, Message-IDs, range-diffs and the like. In addition
to that, b4 also has many other subcommands that help the maintainer and
reviewers.

The Git project uses the same infrastructure as the kernel, so this tool
is also a very good fit for us. Adapt "MyFirstContribution" to
explicitly recommend its use.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>