Document the send_read_only_transaction

[nsjames]

v1/chain/send_readonly_transaction

https://github.com/AntelopeIO/leap/releases/tag/v4.0.0

AC:

• We need a guide describing what this feature is, why it's useful, and how to use it
• Question: What is the diff between this and compute_transaction
• Put into advanced-concepts as a guide

[nsjames]

This was already done in previous tickets