Add documentation

This PR adds documentation infrastructure using readthedocs for hosting the documentation and mkdocs for markdown support.

1. Adds documentation for getting started and POS tagger samples.
2. Adds readthedocs badge to Readme.
3. Ignores the site/ dir generated by mkdocs.

Why mkdocs and readthedocs?
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Very easy to set up, write docs and test them locally before submitting for changes.

Assumptions
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* cl-nlp as project name in readthedocs.org settings

TBD
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1. Import the project in RTD using cl-nlp as name.
2. Set MkDocs as documentation type in project dashboard on RTD.

Reference: https://docs.readthedocs.org/en/latest/getting_started.html#import-your-docs