Adds documents for FastMNMF2

[sekiguchi92]

Thanks for sending a pull request (PR), we really appreciate that! Before hitting the submit button, we'd be really glad if you could make sure all the following points have been cleared.

Please also refer to the doc on contributing for more details. Even if you can't check all the boxes below, do not hesitate to go ahead with the PR and ask for help there.

• [x] Are there docstrings ? Do they follow the numpydoc style ?
• [ ] Have you run the tests by doing nosetests or py.test at the root of the repo ?
• [x] Have you checked that the doc builds properly and that any new file has been added to the repo ? How to do that is covered in the documentation.
• [ ] Is there a unit test for the proposed code modification ? If the PR addresses an issue, the test should make sure the issue is fixed.
• [ ] Last but not least, did you document the proposed change in the CHANGELOG file ? It should go under "Unreleased". (I did not do that because "Added FastMNMF2 ..." is still in Unreleased section")

Happy PR :smiley:

[fakufaku]

The file was missing from the doc! I see, thanks for the PR!