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 +62,14 @@ 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 auto-corrected.
+
+::
+
+    ./scripts-dev/lint.sh
+
 Changelog
 ~~~~~~~~~
 

changelog.d/6164.doc

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

synapse/python_dependencies.py

@@ -97,6 +97,7 @@
     "sentry": ["sentry-sdk>=0.7.2"],
     "opentracing": ["jaeger-client>=4.0.0", "opentracing>=2.2.0"],
     "jwt": ["pyjwt>=1.6.4"],
+    "lint": ["black", "flake8", "isort"],
 }
 
 ALL_OPTIONAL_REQUIREMENTS = set()  # type: Set[str]