Adjust the README to include the steps for getting started 

Specifying exactly how to add this to your Gemfile and run the generator makes it easier for people to get up and running with figaro!

This follows the same format as other gems such as devise, carrierwave, etc.

See more discussion on this change [here](https://github.com/laserlemon/figaro/commit/1f6e7090c71e448db62cd2b1503ed6163998c9f6#commitcomment-6225027).

Let me know if there are any other changes I need to make to help this get merged quickly.  Thanks!