Add missing MySQL2 documentation

[pawelchcki]

MySQL2

The MySQL2 integration traces any SQL command sent through mysql2 gem.

require 'mysql2'
require 'ddtrace'

Datadog.configure do |c|
  c.use :mysql2, options
end

client = Mysql2::Client.new(:host => "localhost", :username => "root")
client.query("SELECT * FROM users WHERE group='x'")

Where options is an optional Hash that accepts the following parameters:

Key	Description	Default
service_name	Service name used for MySQL2 instrumentation	mysql2
tracer	A Datadog::Tracer instance used to instrument the application. Usually you don't need to set that.	Datadog.tracer

[delner]

Looks good! There's one tiny alteration you could make, but you can merge it either way.

[delner]

Minor non blocking but you could wrap the mysql2 with a ` sign.

[pawelchcki]

On it

[pawelchcki]

done()