git: add pre-commit dependency

This also adds documentation for how to install this git pre-commit hook
which will run black before committing. The only downside to doing that
is the, perhaps confusing, initial run which will clone the black
repository before a dev's first commit.

Running on the command-line will be quite obvsious since the default
output shows what pre-commit is doing. But running in a gui or
editor (e.g. magit) will hide that. Viewer discretion is advised.

.pre-commit-config.yaml

@@ -0,0 +1,5 @@
+repos:
+  - repo: https://github.com/ambv/black
+    rev: stable
+    hooks:
+      - id: black

CONTRIBUTING.rst

@@ -130,6 +130,10 @@ our tests)::
 
     $ black --check --exclude=xonsh/ply/ xonsh/ xontrib/
 
+To add this as a git pre-commit hook::
+
+    $ pre-commit install
+
 **********
 Imports
 **********

requirements-docs.txt

@@ -9,3 +9,4 @@ matplotlib
 doctr
 tornado
 black
+pre-commit

requirements-tests.txt

@@ -9,3 +9,4 @@ pygments>=2.2
 codecov
 coverage
 black
+pre-commit