docs: mention MyFirstContribution in more places

While the MyFirstContribution guide exists and has received some use and
positive reviews, it is still not as discoverable as it could be. Add a
reference to it from the GitHub pull request template, where many
brand-new contributors may look. Also add a reference to it in
SubmittingPatches, which is the central source of guidance for patch
contribution.

Signed-off-by: Emily Shaffer <emilyshaffer@google.com>
Reviewed-by: Philippe Blain <levraiphilippeblain@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: nasamuffin

.github/CONTRIBUTING.md

@@ -16,4 +16,7 @@ If you prefer video, then [this talk](https://www.youtube.com/watch?v=Q7i_qQW__q
 might be useful to you as the presenter walks you through the contribution
 process by example.
 
+Or, you can follow the ["My First Contribution"](https://git-scm.com/docs/MyFirstContribution)
+tutorial for another example of the contribution process.
+
 Your friendly Git community!

Documentation/SubmittingPatches

@@ -3,8 +3,9 @@ Submitting Patches
 
 == Guidelines
 
-Here are some guidelines for people who want to contribute their code
-to this software.
+Here are some guidelines for people who want to contribute their code to this
+software. There is also a link:MyFirstContribution.html[step-by-step tutorial]
+available which covers many of these same guidelines.
 
 [[base-branch]]
 === Decide what to base your work on.