Add tips for the changelog to the pull request template (#6663)

.github/PULL_REQUEST_TEMPLATE.md

@@ -3,6 +3,10 @@
 <!-- Please read CONTRIBUTING.md before submitting your pull request -->
 
 * [ ] Pull request is based on the develop branch
-* [ ] Pull request includes a [changelog file](https://github.com/matrix-org/synapse/blob/master/CONTRIBUTING.md#changelog)
+* [ ] Pull request includes a [changelog file](https://github.com/matrix-org/synapse/blob/master/CONTRIBUTING.md#changelog). The entry should:
+  - Be a short description of your change which makes sense to users. "Fixed a bug that prevented receiving messages from other servers." instead of "Moved X method from `EventStore` to `EventWorkerStore`.".
+  - Use markdown where necessary, mostly for `code blocks`.
+  - End with either a period (.) or an exclamation mark (!).
+  - Start with a capital letter.
 * [ ] Pull request includes a [sign off](https://github.com/matrix-org/synapse/blob/master/CONTRIBUTING.md#sign-off)
 * [ ] Code style is correct (run the [linters](https://github.com/matrix-org/synapse/blob/master/CONTRIBUTING.md#code-style))

CONTRIBUTING.md

@@ -101,8 +101,8 @@ in the format of `PRnumber.type`. The type can be one of the following:
 The content of the file is your changelog entry, which should be a short
 description of your change in the same style as the rest of our [changelog](
 https://github.com/matrix-org/synapse/blob/master/CHANGES.md). The file can
-contain Markdown formatting, and should end with a full stop ('.') for
-consistency.
+contain Markdown formatting, and should end with a full stop (.) or an
+exclamation mark (!) for consistency.
 
 Adding credits to the changelog is encouraged, we value your
 contributions and would like to have you shouted out in the release notes!

changelog.d/6663.doc

@@ -0,0 +1 @@
+Add some helpful tips about changelog entries to the github pull request template.