Add light client implementation guide

[colin-axner]

Summary

A guide to implementing a light client for IBC

• overview (vocabulary - client/consensus states, client message etc ie each interface) light client guide: concepts/vocabulary #1850
• client_state.go and its functions light client guide: client state functions  #1851
• consensus_state.go and its functions light client guide: consensus state functions #1852
• existence/non-existence proofs light client guide: existence/non-existence proofs #1853
• update function light client guide: updates and misbehaviour handling #1854
• misbehaviour handling light client guide: updates and misbehaviour handling #1854
• upgrade handling
• proposal handling
• genesis metadata

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[colin-axner]

Must note light client requirements such as:

• checking for misbehaviour on updates and freezing the client if misbehaviour is found
• checking for duplicate updates
• accounting for time and block delay

[colin-axner]

Every client state interface function should have a section and clearly document the requirements/expectations of that 02-client has for those functions.

Explanation of types ClientState, ConsensusState, ClientMessage is needed. How misbehaviour submission works via MsgUpdateClient