Add clear contributor docs to canonical location

[lukpueh]

The in-toto Python implementation seems to lack a clear contributor quick start guide. This can be easily fixed:

• create docs/CONTRIBUTING.md, which is the canonical place according to GitHub Community Standards
• point to dev installation instructions in https://in-toto.readthedocs.io/en/latest/installing.html#installing-for-development
• mention the tox command and DCO
• include in RTD TOC

As a start the whole thing could look very similar to https://theupdateframework.readthedocs.io/en/latest/CONTRIBUTING.html until (excluding) the "Unit Test" section

[lukpueh]

other things to add

• running the test suite manually, running individual tests, coverage... (this is similar to instructions for theupdateframework above, except that the test suite aggregation script is called runtests.py)
• auto format with black and isort

  $ black .
  $ isort .
  

• build docs

  $ cd doc && make html 
  

[DarikshaAnsari]

@lukpueh I have added contributing.md to docs repo. PTAL !