Document how to stub route helper functions in isolated tests. Addresses...

... #506.

See the discussion in #506 about the issue this is addressing.

I'm happy to revise this however to best fit into the existing documentation. I'm also not entirely clear if this is a minitest specific thing or if it affects rspec as well. I'd also be fine with changing from mocha's `stubs` in the example to rspec's `stub` if readers of this documentation are more likely to be familiar with rspec's stubbing stuff.

I'm also cool with moving this to the wiki somewhere if this doesn't belong in the README.

Basically tell me how to change this and i'll be glad to do it :)