Add new developer guide with instructions on using pre-commit.

docs/developer-guide.rst

@@ -0,0 +1,44 @@
+Developer Guide
+===============
+
+
+Pre-commit Checks
+-----------------
+
+This project is configured for use with `pre-commit <https://pre-commit.com>`_ to perform some static code analysis checks. By default, pre-commit behavior is disabled. To enable pre-commit in a local checkout, first install pre-commit (consider using `pipx <https://pipxproject.github.io/pipx/>`_), then install the hooks with ``pre-commit install``.
+
+For example::
+
+	tahoe-lafs $ pre-commit install
+	pre-commit installed at .git/hooks/pre-commit
+	tahoe-lafs $ echo "def foo():\n\t''" > src/allmydata/tabbed.py
+	tahoe-lafs $ git add src/allmydata/tabbed.py
+	tahoe-lafs $ git commit -a -m "Add a file that violates flake8"
+	flake8...................................................................Failed
+	- hook id: flake8
+	- exit code: 1
+
+	src/allmydata/tabbed.py:2:1: W191 indentation contains tabs
+
+To uninstall::
+
+	tahoe-lafs $ pre-commit uninstall
+	pre-commit uninstalled
+
+
+Some find running linters on every commit to be a nuisance. To avoid the checks triggering during commits, but to check before pushing to the CI, install the hook for pre-push instead::
+
+	tahoe-lafs $ pre-commit install -t pre-push
+	pre-commit installed at .git/hooks/pre-push
+	tahoe-lafs $ git commit -a -m "Add a file that violates flake8"
+	[3398.pre-commit 29f8f43d2] Add a file that violates flake8
+	 1 file changed, 2 insertions(+)
+	 create mode 100644 src/allmydata/tabbed.py
+	tahoe-lafs $ git push
+	flake8...................................................................Failed
+	- hook id: flake8
+	- exit code: 1
+
+	src/allmydata/tabbed.py:2:1: W191 indentation contains tabs
+
+	error: failed to push some refs to 'github.com:jaraco/tahoe-lafs.git'

docs/index.rst

@@ -39,6 +39,8 @@ Contents:
    write_coordination
    backupdb
 
+   developer-guide
+
    anonymity-configuration
 
    nodekeys