Add 'Getting Started' message to README.rst
Rephrase instructional code

- Now, instead of explicitly saying "you only need to do this once", I say for
  each command when you need to do it, using words like "each" and "the first
  time"

- More explanation of the Debian/Ubuntu install problem

- Remove needless warning for people using nonstandard shells.  (The
  instructions work correctly for bash, and with slight modification for zsh
  as well.  This covers all reasonable users.)
Only pip install once

As Ian/zenhack noted, the '-e' flag to 'pip' makes the install editable, so
only one install is required.
Change formatting of 'haas' in README.rst
Merge pull request #117 from gsilvis/doc-update-getting-started

Add 'Getting Started' message to README.rst