Document lint.sh & allow application to specified files only (#6312)

CONTRIBUTING.rst

@@ -58,6 +58,14 @@ All Matrix projects have a well-defined code-style - and sometimes we've even
 got as far as documenting it... For instance, synapse's code style doc lives
 at https://github.com/matrix-org/synapse/tree/master/docs/code_style.md.
 
+To facilitate meeting these criteria you can run ``scripts-dev/lint.sh``
+locally. Since this runs the tools listed in the above document, you'll need
+python 3.6 and to install each tool. **Note that the script does not just
+test/check, but also reformats code, so you may wish to ensure any new code is
+committed first**. By default this script checks all files and can take some
+time; if you alter only certain files, you might wish to specify paths as
+arguments to reduce the run-time.
+
 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.

changelog.d/6312.misc

@@ -0,0 +1 @@
+Document the use of `lint.sh` for code style enforcement & extend it to run on specified paths only.

scripts-dev/lint.sh

@@ -7,7 +7,15 @@
 
 set -e
 
-isort -y -rc synapse tests scripts-dev scripts
-flake8 synapse tests
-python3 -m black synapse tests scripts-dev scripts
+if [ $# -ge 1 ]
+then
+  files=$*
+else
+  files="synapse tests scripts-dev scripts"
+fi
+
+echo "Linting these locations: $files"
+isort -y -rc $files
+flake8 $files
+python3 -m black $files
 ./scripts-dev/config-lint.sh