Add rule to favor interpolation over concatenation

Strings should not be concatenated, and instead be assembled with
interpolation. Technically, it is faster for ruby to process because
interpolation modifies the one string, and concatenation creates a new
string from the two being added - then points to the new one.
Merge pull request #1 from reachlocal/add_interpolation_rule

Add rule to favor interpolation over concatenation
Add information about having equal signs align

There was not source of truth whether to align equal signs or not
in our code. This should help people not align equal signs as well
as help them rethink their code if they have many assignments.
Simplified rule and changed example/format

I added a note for the code smell. We don't have a rule to link to
yet regarding too many assignments. That rule seems more related to
OOP rather than just format and styling within Ruby. I have an escaped
asterisk because it was causing issues with VIM syntax highlighting.
Also added a new line to try and stop the error message about missing a
newline at end of file.