Add CONTRIBUTING.md. Resolves #274

CONTRIBUTING.md

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+# Contributing
+
+## Local development
+
+- The tests depend on having at least the following installed (possibly not
+  a complete list)
+  - git (A sufficiently newer version is required to run pre-push tests)
+  - python
+  - python3.3 (Required by a test which checks different python versions)
+  - python3.4 (Required by a test which checks different python versions)
+  - tox (or virtualenv)
+  - ruby + gem
+
+### Setting up an environemnt
+
+This is useful for running specific tests.  The easiest way to set this up
+is to run:
+
+1. `tox -e venv`
+2. `. venv-pre_commit/bin/activate`
+
+This will create and put you into a virtualenv which has an editable
+installation of pre-commit.  Hack away!  Running `pre-commit` will reflect
+your changes immediately.
+
+### Running a specific test
+
+Running a specific test with the environment activated is as easy as:
+`py.test tests -k test_the_name_of_your_test`
+
+### Running all the tests
+
+Running all the tests can be done by running `tox -e py27` (or your
+interpreter version of choice).  These often take a long time and consume
+significant cpu while running the slower node / ruby integration tests.
+
+Alternatively, with the environment activated you can run all of the tests
+using:
+`py.test tests`
+
+To skip the slower ruby / node integration tests, you can set the environment
+variable `slowtests=false`.
+
+### Setting up the hooks
+
+With the environment activated simply run `pre-commit install`.
+
+## Style
+
+This repository follows pep8 (and enforces it with flake8).  There are a few
+nitpicky things I also like that I'll outline below.
+
+### Multi-line method invocation
+
+Multiple line method invocation should look as follows
+
+```python
+function_call(
+    argument,
+    argument,
+    argument,
+)
+```
+
+Some notable features:
+- The intial parenthese is at the end of the line
+- Parameters are indented one indentation level further than the function name
+- The last parameter contains a trailing comma (This helps make `git blame`
+  more accurate and reduces merge conflicts when adding / removing parameters).
+
+## Documentation
+
+Documentation is hosted at http://pre-commit.com
+
+This website is controlled through
+https://github.com/pre-commit/pre-commit.github.io
+
+When adding a feature, please make a pull request to add yourself to the
+contributors list and add documentation to the website if applicable.