Docs/fix for rails 6.0

[sgringwe]

Hoping to contribute some small improvements related to rails routing on older rails version 6.0. I ran into a few errors in a rails 6.0.x app, hopefully this saves some pain for others.

https://github.com/rails/rails/blob/6-1-stable/actionpack/CHANGELOG.md and https://github.com/rails/rails/pull/37892/files

The errors I got were:

• Error that the @app.paths['config/routes'] is nil. This is "fixed" by skipping that path or any other nil paths when injecting paths
• Error that draw function is undefined when calling it in root routes.rb file. I added a note in README about requiring rails 6.1 for this function.

Open to feedback on approach

[sgringwe]

@ngan ptal!

[sgringwe]

@ngan @technicalpickles ptal :)