Adding contributor guidelines.

CONTRIBUTING.md

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+Contributing to ActivitySim
+===========================
+
+Style
+-----
+
+- Python code should follow the [PEP 8 Style Guide][pep8].
+- Python docstrings should follow the [NumPy documentation format][numpydoc].
+
+### Imports
+
+Imports should be one per line.
+Imports should be grouped into standard library, third-party,
+and intra-library imports. `from` import should follow "regular" `imports`.
+Within each group the imports should be alphabetized.
+Here's an example:
+
+```python
+import sys
+from glob import glob
+
+import numpy as np
+
+import urbansim.urbansim.modelcompile as modelcompile
+from urbansim.util import misc
+```
+
+Imports of scientific Python libraries should follow these conventions:
+
+```python
+import matplotlib.pyplot as plt
+import numpy as np
+import pandas as pd
+import scipy as sp
+```
+
+
+Thanks!
+
+[pep8]: http://legacy.python.org/dev/peps/pep-0008/
+[numpydoc]: https://github.com/numpy/numpy/blob/master/doc/HOWTO_DOCUMENT.rst.txt