Rewrite readme (#294)

This mainly reorganizes the readme based on questions and feedback from
users on GitHub over the years:

- Getting Started: Short section which covers the main use case and
  doesn't go into details.
  - Pip command at the very beginning so that users are less likely to
    mistakenly install another package.
  - Basic application code to load the .env file into the environment.
  - Introduction to the syntax of .env files with a short example.
- Other common use cases:
  - Load configuration without altering the environment
  - Parse configuration as a stream
  - Load .env files in IPython
- Command-line Interface
- File format: Details about the syntax of .env files, previously
  scattered around.
- Related Projects: I'm not sure we really need that one but I guess we can keep
  it for now.
- Acknowledgements

Minor changes:

- I removed the "saythanks" link since it is dead.
- I removed the banner made in ASCII art since it read ".env" and not
  "python-dotenv", which I found distracting.  We could make another one
  but I don't have time right now.  It also saves the user some
  scrolling.

Author: bbc2