For the current release, Travis CI and Coveralls status.

Closes #2186.

Also update command specifications to match descriptions.

GitHub now nicely generates a documentation site for us at
http://brew.sh/brew based on our docs folder. Optimise the output of
this and the GitHub docs directory for readability and the various user
groupings.

Addressing a few spots where I looked for terminology and did not find what I was looking for. Input based on a recent [podcast with Mike](https://changelog.com/podcast/223)

Closes #1040.

Always a good idea to quote command substitution.

Also, explicitly welcome new/beginner contributors and give them a suggested
starting point to contribute to Homebrew. Based on the boiler plate I tend to
send people to for GSoC, Outreachy or generally people who say "I want to try
and contribute to open-source".