Update README.md

Setup

In order for methods like 1.meter to work, you'll either need to setup Alchemist yourself.

Add the following line to your config/initializers/alchemist.rb:

Alchemist.setup # This will load every category of measurement
If you only want to use one category for conversions you can load it individually:

Alchemist.setup('distance') # This will load only distance
Restart your application.