Write `claim` module guide

[lumtis]

Depends on https://github.com/tendermint/spn/issues/863

As we want the projects to import this module, we should make public a guide (README in the directory of the module) that describes how to use it To write down:

• Disclaimer that the module is not fully audited
• Module principles (claim records, missions, decay)
• How a mission is completed
• What is the initial claim
• How to claim for staking and voting
• How to add new claims
  • With direct CompleteMission call (bad practice)
  • With hooks (good practice)
• Integrate simulation tests

[lumtis]

transfered