Document changelog fragment format. (#11004)

CONTRIBUTING.md

@@ -15,12 +15,12 @@ talk to us! You can find out our thoughts and ensure that your contribution
 won't clash or be obviated by Vault's normal direction. A great way to do this
 is via the [Vault Discussion Forum][2].
 
-This document will cover what we're looking for in terms of reporting issues.
+## Issues
+
+This section will cover what we're looking for in terms of reporting issues.
 By addressing all the points we're looking for, it raises the chances we can
 quickly merge or address your contributions.
 
-## Issues
-
 ### Reporting an Issue
 
 * Make sure you test against the latest released version. It is possible
@@ -61,6 +61,30 @@ quickly merge or address your contributions.
    the issue tracker clean. The issue is still indexed and available for
    future viewers, or can be re-opened if necessary.
 
+## Pull requests
+
+Please include a file within your PR named `changelog/#.txt`, where `#` is your
+pull request ID.  There are many examples under [changelog](changelog/), but
+the general format is
+
+````
+```release-note:CATEGORY
+COMPONENT: summary of change
+```
+````
+
+CATEGORY is one of `security`, `change`, `feature`, `improvement`, or `bug`.
+Your PR is almost certain to be one of `bug` or `improvement`, but don't
+worry too much about getting it exactly right, we'll tell you if a change is 
+needed.
+
+To determine the relevant component, consult [CHANGELOG](CHANGELOG.md) and pick
+whichever one you see that seems the closest match.
+
+You do not need to include the link at the end of the summary that appears in
+CHANGELOG.md, those are generated automatically by the changelog-building 
+process.
+
 ## Setting up Go to work on Vault
 
 If you have never worked with Go before, you will have to complete the