Add Pikaday gem to README

Thank you so much for creating the pikaday datepicker.
Since we're working on a Ruby on Rails project we've created an open-source gem which bundles it.

You can find it at https://rubygems.org/gems/pikaday-gem
Source code is at https://github.com/ets-berkeley-edu/pikaday-gem
And readme at https://github.com/ets-berkeley-edu/pikaday-gem#readme

It relies on the fact that you're tagging new versions. So far there is only `1.0.0` but we're already looking forward to the next one.

We'll also version our gem in the following way.
The first 3 (x.x.x) will be the same as your version, while the last digit (_._._.x) will be gem updates.

Hope you'll like it.