Followup fixups for PR #2.

This introduces a lot of small followup changes to
https://github.com/prometheus/docs/pull/2.

Some things are clear right/wrong spelling/grammar issues, while others
are a matter of style or taste.

Except in extreme cases, I didn't reflow the text aggressively to a
certain number of columns, in order to make it easier to spot
differences.

Let me know what you think.

Some reference links worth mentioning:

Compound adjectives:
http://www.grammar-monster.com/lessons/adjectives_compound_adjectives.htm

Run-on sentences:
http://grammar.ccc.commnet.edu/grammar/runons.htm

General tech-writing style guide (which I haven't actually looked at
much yet):
http://infobits.eu/tech-doc/style-guide/

Add lots of docs, mostly best practices.

Start out on the visualisation docs.

Expand overview, remove federation as a feature we don't have that yet.

Switch example variable to follow naming scheme.

Add Dockerfile