Add high-level docs for the Ruby gem

[composerinteralia]

#71 was a good reminder that the gem does a whole bunch of things that currently have little or no documentation. With Trilogy soon to be an officially supported option for Rails, now is a good time to add some better documentation (expanding what we have in https://github.com/github/trilogy/blob/7ceee303b7d440f0b26cba65d00032bfb897c0ef/contrib/ruby/README.md would be a good place to start).

[bensheldon]

Some things that we likely want to document, either through example or reasonable reference (i.e. anything that is Trilogy-specific, or database options/flags that are translated into Ruby-style; we shouldn't duplicately list every database options/flag that are documented elsewhere):

• Trilogy.new arguments
• Trilogy instance methods (the practical example already in the readme is a good starting place)
• Usage of Trilogy::Result
• Error classes and their general meaning