Add BEEFY specifications

[bhargavbh]

The current BEEFY spec is a stub. Update the BEEFY specifications, including the consensus components and Subsampling based Light-Client verifier.

• [ ] Protocol overview
• [ ] Consensus layer for Node :
  1. MMR
  2. Core Consensus for following Grandpa:
     • Initial sync/ bootstrap
     • Next Beefy Round selection
     • Playing a Beefy Round
     • Beefy Justifications

3. Runtime Pallets and interactions
   • [ ] Verification by Subsampling based Light Client

[bhargavbh]

Closed in https://github.com/w3f/polkadot-spec/pull/682