Provide build instructions

Version 3 may be on the road to Grunt (#163) but while we still have the Rakefile the README should have a little advice on how to build.

I found two issues.

The Rakefile uses Ruby 1.9's new hash syntax, so running `rake` in 1.8 (still the default on Mac) will throw up this error:

```
rake aborted!
/Users/cjoslyn/Code/underscore.string/Rakefile:2: syntax error, unexpected ':', expecting $end
task default: :test
^
```

If you know of a nice set of ruby 1.9 installation instructions, please link to them. There are probably at least a few [gotchas](http://stackoverflow.com/a/8237833/199644).

Also, since you're using Gemfile.lock, take care to remind intrepid builders that in order to use the right versions of everything they should run `bundle install` and to use `bundle exec rake` instead of `rake`.