doc: linkable commit message guidelines

Put the commit guidelines themselves under a heading to be more
prominent, and to allow linking directly to them (instead of the section
on how to use git).

Link the pull request template to the guidelines, so contributors can
find them.

PR-URL: #11792
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>

.github/PULL_REQUEST_TEMPLATE.md

@@ -13,7 +13,9 @@ Contributors guide: https://github.com/nodejs/node/blob/master/CONTRIBUTING.md
 - [ ] `make -j4 test` (UNIX), or `vcbuild test` (Windows) passes
 - [ ] tests and/or benchmarks are included
 - [ ] documentation is changed or added
-- [ ] commit message follows commit guidelines
+- [ ] commit message follows [commit guidelines][]
 
 ##### Affected core subsystem(s)
 <!-- Provide affected core subsystem(s) (like doc, cluster, crypto, etc). -->
+
+[commit guidelines]: https://github.com/nodejs/node/blob/master/CONTRIBUTING.md#commit-guidelines

CONTRIBUTING.md

@@ -95,6 +95,8 @@ $ git add my/changed/files
 $ git commit
 ```
 
+### Commit guidelines
+
 Writing good commit logs is important. A commit log should describe what
 changed and why. Follow these guidelines when writing one: