Improve instructions on how to write CHANGELOG entries

This makes easier for contributors to get the CHANGELOG entry
right the first time.

.github/PULL_REQUEST_TEMPLATE.md

@@ -11,5 +11,13 @@ Here is a quick checklist that should be present in PRs.
 Unless your change is trivial or a small documentation fix (e.g., a typo or reword of a small section) please:
 
 - [ ] Create a new changelog file in the `changelog` folder, with a name like `<ISSUE NUMBER>.<TYPE>.rst`. See [changelog/README.rst](https://github.com/pytest-dev/pytest/blob/master/changelog/README.rst) for details.
+
+  Write sentences in the **past or present tense**, examples:
+
+  * *Improved verbose diff output with sequences.*
+  * *Terminal summary statistics now use multiple colors.*
+
+  Also make sure to end the sentence with a `.`.
+
 - [ ] Add yourself to `AUTHORS` in alphabetical order.
 -->

changelog/README.rst

@@ -1,12 +1,14 @@
 This directory contains "newsfragments" which are short files that contain a small **ReST**-formatted
 text that will be added to the next ``CHANGELOG``.
 
-The ``CHANGELOG`` will be read by users, so this description should be aimed to pytest users
+The ``CHANGELOG`` will be read by **users**, so this description should be aimed to pytest users
 instead of describing internal changes which are only relevant to the developers.
 
-Make sure to use full sentences with correct case and punctuation, for example::
+Make sure to use full sentences in the **past or present tense** and use punctuation, examples::
 
-    Fix issue with non-ascii messages from the ``warnings`` module.
+    Improved verbose diff output with sequences.
+
+    Terminal summary statistics now use multiple colors.
 
 Each file should be named like ``<ISSUE>.<TYPE>.rst``, where
 ``<ISSUE>`` is an issue number, and ``<TYPE>`` is one of: