Add support for pre-commit-hooks

This will help forgetful developers like me not to miss running tox -e
pep8 before commiting.

Change-Id: Ied40f0daa59d9de34867c7b0cc4897a9332a7413
Signed-off-by: Antoni Segura Puimedon <celebdor@gmail.com>

.pre-commit-config.yaml

@@ -0,0 +1,6 @@
+repos:
+    - repo: https://github.com/pre-commit/pre-commit-hooks
+      rev: v1.4.0
+      hooks:
+          - id: flake8
+

CONTRIBUTING.rst

@@ -15,3 +15,12 @@ Pull requests submitted through GitHub will be ignored.
 Bugs should be filed on Launchpad, not GitHub:
 
    https://bugs.launchpad.net/kuryr-kubernetes
+
+If you want to have your code checked for pep8 automatically before commiting
+changes, you can just do::
+
+    pip install pre-commit
+    pre-commit install
+
+From that moment on, every time you run *git commit* it will first check your
+diff for pep8 compliance and refuse to commit if it doesn't pass.