Document all available attributes

[glindstedt]

All available attributes should be documented in the main README.md to avoid confusion. So far I'm aware of:

• data in _jsonnet_common_attrs
• jsonnet in _jsonnet_common_attrs
• extra_args in _jsonnet_compile_attrs

I lost several hours trying to work around not having the data attribute in a jsonnet_library rule, because it was not documented and I assumed the documentation was complete. I only found out it was already implemented when I was looking at the code with the intent of adding it myself.

If this seems reasonable I can help out drafting a PR

Related to #165