self.load .tt extension documentation in README

[Nicholas Palaniuk]

I was just following along in the README and it might be worth adding if you use the .tt extension for your grammar file than you must specify .tt in Treetop.load 'my_grammar.tt' because it defaults to a .treetop extension

[Lucas Neves Martins]

Don't be shy, send a PR 😄

[Dewayne VanHoozer]

https://github.com/cjheath/treetop/blob/master/lib/treetop/polyglot.rb

[Clifford Heath]

Processed the PR.