Updated README.md for readability and context
Updated README.md for readability and context

Summary:
Updated README.md to accomplish better readability without logical interruptions and provide background on the project.

- Table of Contents is moved above "Three things [..]" statement.
  Here is one reason: it's weird when you have a colon at the end
  of a sentence and then context completely changes. If you're about
  to describe 3 things, make sure they are right next to that sentence
- Table of Contents is re-formatted to reflect the heading structure
  of the README article. The ## headings are at the root of the list
  and the ### headings are on the second level.
- A short statement is added clarifying the fact of fbthrift's uniqueness,
  its roots, and relationship to the Apache Thrift project. This is done
  based on the text of the original FB Code release blog and links to it and
  the Apache Thrift distribution have been added accordingly.
Closes https://github.com/facebook/fbthrift/pull/257

Reviewed By: spalamarchuk

Differential Revision: D6722011

Pulled By: yfeldblum

fbshipit-source-id: be31602bfe8e3d9032a723bf92ea4607f9107693

facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift