Document all available attributes

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

Start with the main README.md and inspect the `_jsonnet_common_attrs` and `_jsonnet_compile_attrs` definitions, including the listed `data`, `jsonnet`, and `extra_args` attributes. Document every available attribute in the README, and consider the work complete when the documented list matches the implemented attributes.

Written by the indexing model from the issue text.