Update CONTRIBUTING.md

Author: alexbw

CONTRIBUTING.md

@@ -17,4 +17,22 @@ Follow either of the two links above to access the appropriate CLA and instructi
 
 ### Contributing code
 
-If you have improvements to Tangent, send us your pull requests!
+#### Adding new derivatives
+
+We still have a lot of derivatives we need to write! To add a new derivative for a primitive operation,
+
+- [Read the docs on how to write derivatives in Tangent, and look at some examples](https://github.com/google/tangent/blob/7bf4eaffd646a5906aa15a852f117833d37fb09a/tangent/grads.py#L14-L33)!
+- Add your primitive op's reverse-mode derivative to grads.py (example reverse-mode derivative [for np.sin](https://github.com/google/tangent/blob/7bf4eaffd646a5906aa15a852f117833d37fb09a/tangent/grads.py#L230-L232), and for [tf.sin](https://github.com/google/tangent/blob/7bf4eaffd646a5906aa15a852f117833d37fb09a/tangent/tf_extensions.py#L183-L185)).
+- Add its forward-mode derivative to tangents.py (example forward-mode derivative for [np.sin](https://github.com/google/tangent/blob/7bf4eaffd646a5906aa15a852f117833d37fb09a/tangent/tangents.py#L144-L146), and for [tf.sin](https://github.com/google/tangent/blob/7bf4eaffd646a5906aa15a852f117833d37fb09a/tangent/tf_extensions.py#L344-L346))
+- Add a function using the primitive operation in functions.py. Our tests will pick up on it automatically ([example test function](https://github.com/google/tangent/blob/7bf4eaffd646a5906aa15a852f117833d37fb09a/tests/functions.py#L406-L407)).
+- Make sure the tests pass. The tests will be run automatically with Travis once you submit a PR, but it's good to do this locally, so you don't have to wait as long.
+```
+# Make sure you have pytest installed
+pip install pytest
+# Run this command from the root of the Tangent project
+py.test --short tests
+```
+
+#### Adding other new functionality
+
+Tangent is a work-in-progress, so there's a lot of upgrades and tweaks that would be useful. If you've already fixed a bug, or added an enhancement, open a PR, and the team will take a look at it, and help you polish it, conform to style guidelines etc., before we merge it. If you're thinking about embarking on a feature enhancement, open a GitHub issue to start a discussion, or [talk to us on gitter](https://gitter.im/google/tangent).