Update readme

[juanmanuelramallo]

Why

We want to let users know how to use this gem

What

Update readme file with the following information:

• Table of contents: Displaying links to each section in the readme
• Getting started: Explaining how to install and use this gem
• Motivation: Explaining why to use this pattern
• Real life examples: One or two examples showcasing the usage of the gem in a real life application
• Notes: In order to support major form helpers gems like SimpleForm or even plain rails form helpers, we would want to define the methods:
  • persisted? => to let the gem know if the form object being saved is a new record or not
  • model_name => which will let the gem know where to look for translations model_name should return an ActiveModel::Name instance, most likely to be the model_name of the main model being saved.
• Useful links: A list of links where the form object pattern is explained

https://docs.google.com/spreadsheets/d/1nilkCK2ZezTwR7rz6lazNG-5D-muK3WutOX4pps6zC4/edit#gid=2109604323