Add internal REST API documentation

* I added `docs` directory, which consists of internal REST API
  documentation, in the root directory.
* I added `requirements.txt` for `pip`, which is used for installing
  required modules. It is recommended to install those modules in
  a virtualenv environment.
* I added a README file for `docs` directory.
* I modified `.gitignore` file to ignore built documentation and
  virtualenv environments in the project directory.