Link to CONTRIBUTING.md in README
Pick random port when generating Rails app

We have lots of Suspenders-generated apps. We run them using Foreman.
Randomizing the port number can help us run multiple apps locally while avoiding
conflicts.

Convert `bin_setup` file to use ERb and interpolate `port_number` into template.
Add i18n-tasks for missing/unused translations

* i18n-tasks looks for missing *or* unused translations
* Add config file to ignore certain i18n keys (eg: activerecord) which
  are used as defaults but not explicitly in views or helpers
* Remove code to raise in test env when there are unused translations