updated readme to include info on creating source maps and other bundle options

I added a bit more info to the README about how to generate source maps with a link to the bundle options section of the Browserify source code.

I spent a bit of time tracking down how to generate source maps properly and finally found the answer in a closed Github issue.  I think being explicit about where `bundle` options can be found would be beneficial for people, especially those new to Browserify.