Add Documentation

[keon]

This is an extension of issue #264
To prepare for packaging, we need to properly explain how api works in our documentation.

[goswami-rahul]

@keon should we show the API usage in each module's (or each file's) docstring, or do we write a separate API documentation?
We also need to have complete docstrings in each file to explain what problem the algorithm is solving.

[keon]

@goswami-rahul
Usually we write api usage in docstring and automatically compile it into docs.
And you're right. we have to write docstrings in the right format in each algorithm.