Add documentation for VCS hooks

Closes #249

docs/source/user/index.rst

@@ -26,6 +26,7 @@ This guide will cover all of these and the nuances for using |Flake8|.
     error-codes
     ignoring-errors
     using-plugins
+    using-hooks
     python-api
 
 .. config files

docs/source/user/using-hooks.rst

@@ -0,0 +1,69 @@
+=============================
+ Using Version Control Hooks
+=============================
+
+|Flake8| can be integrated into your development workflow in many ways. A
+default installation of |Flake8| can install pre-commit hooks for both
+`Git`_ and `Mercurial`_. To install a built-in hook, you can use the
+:option:`flake8 --install-hook` command-line option. For example, you can
+install a git pre-commit hook by running:
+
+.. prompt:: bash
+
+    flake8 --install-hook git
+
+This will install the pre-commit hook into ``.git/hooks/``. Alternatively,
+you can install the mercurial commit hook by running
+
+.. prompt:: bash
+
+    flake8 --install-hook mercurial
+
+
+Preventing Commits
+==================
+
+By default, |Flake8| does not prevent you from creating a commit with these
+hooks. Both hooks can be configured to be strict easily.
+
+Both our Git and Mercurial hooks check for the presence of ``flake8.strict``
+in each VCS' config. For example, you might configure this like so:
+
+.. prompt:: bash
+
+    git config --bool flake8.strict true
+    hg config flake8.strict true
+
+
+Checking All Modified Files Currently Tracked
+=============================================
+
+.. note::
+
+    Mercurial does not have the concept of an index or "stage" as best as I
+    understand.
+
+|Flake8| aims to make smart choices that keep things fast for users where
+possible. As a result, the |Flake8| Git pre-commit will default to only
+checking files that have been staged (i.e., added to the index). If, however,
+you are keen to be lazy and not independenty add files to your git index, you
+can set ``flake8.lazy`` to ``true`` (similar to how you would set
+``flake8.strict`` above) and this will check all tracked files.
+
+This is to support users who often find themselves doing things like:
+
+.. prompt:: bash
+
+    git commit -a
+
+.. note::
+
+    If you have files you have not yet added to the index, |Flake8| will not
+    see these and will not check them for you. You must ``git-add`` them
+    first.
+
+
+.. _Git:
+    https://git-scm.com/
+.. _Mercurial:
+    https://www.mercurial-scm.org/