Added reference documentation policy

[DarikshaAnsari]

Fixes #28

[lukpueh]

Very nice! IMO we don't need to refer to the docs setup used by the Python in-toto implementation, as It's very Python-specific. But referring to the contents makes sense.

[lukpueh]

I think there was a misunderstanding. I specifically suggested to not include Python-specific instructions. If you remove the last commit, we can merge this. :)

[DarikshaAnsari]

The previous commit is for the issue #755, these both issue are same so i thought to resolve these in one PR.

[lukpueh]

Okay. Did you see my comments in #755? I transferred the issue to in-toto/in-toto and suggested to add Python in-toto docs guidelines to the Python in-toto repo. A PR against https://github.com/in-toto/in-toto/blob/develop/doc/source/CONTRIBUTING.md with the contents from your last commit here would be appreciated.

[DarikshaAnsari]

Okay I got your point now i will revert this last commit then.

[lukpueh]

Thanks!