Add contributer docs for using the provided linters script

.github/PULL_REQUEST_TEMPLATE.md

@@ -5,3 +5,4 @@
 * [ ] Pull request is based on the develop branch
 * [ ] Pull request includes a [changelog file](https://github.com/matrix-org/synapse/blob/master/CONTRIBUTING.rst#changelog)
 * [ ] Pull request includes a [sign off](https://github.com/matrix-org/synapse/blob/master/CONTRIBUTING.rst#sign-off)
+* [ ] Code style is correct (run the [linters](https://github.com/matrix-org/synapse/blob/master/CONTRIBUTING.rst#code-style))

CONTRIBUTING.rst

@@ -62,6 +62,17 @@ Please ensure your changes match the cosmetic style of the existing project,
 and **never** mix cosmetic and functional changes in the same commit, as it
 makes it horribly hard to review otherwise.
 
+Before doing a commit, ensure the changes you've made don't produce
+linting errors. You can do this by running the linters as follows. Ensure to
+commit any files that were corrected.
+
+::
+    # Install the dependencies
+    pip install -U black flake8 isort
+
+    # Run the linter script
+    ./scripts-dev/lint.sh
+
 Changelog
 ~~~~~~~~~
 

changelog.d/6164.doc

@@ -0,0 +1 @@
+Contributor documentation now mentions script to run linters.