Build out contributing guidelines

[seanpmorgan]

In an effort to encourage involvement, and standardize contributions we should make a concerted effort to build thorough docs on how to contribute for each sub-package (layers, crf, etc.)

For example:
Building from #19 dicussion. We should create standards for adding custom keras Layers. Some things to include:

• Testing with the Keras layer test
• Registering the Layer so a Model can be serialized / deserialized

[facaiy]

@fchollet @anj-s @martinwicke As shown in PR #19, tf.keras cannot recognize custom layers without object_scope, do you have any good advice: how to integrate keras with addons.layers seamlylessly?

[martinwicke]

We should maybe think about a package-spanning registration mechanism. I think that would require a design, and feedback from the Keras community. I can certainly imagine this working via directories, or via calling some registration function on import.

Until then, I think passing the objects is a reasonable fallback.

[seanpmorgan]

Initial pass of this is complete. We'll continue to iterate on the docs though.