Add links on how to install pip

The most common problem, based on submissions to Reddit,
encounted by users is how to install PRAW. They don't
have pip installed and encounter difficulties installing
it. By adding links to tutorials detailing how to install
pip, the number of people having problems with installing
PRAW should decrease.
Minor documentation fixes

 * Add punctuation to the end of all lines, save those that
       are bullet point listings.
 * Add to CHANGES.rst that distinguish can also be used to
       distinguish as special.
 * Keep consistent spacing in CHANGES with 1 line above and
       below each release.
 * Removal of unnecessary empty lines.