Document how to estimate deposit for `set`

[ailisp]

It is helpful to document a formula to precisely count the storage deposit. Used for frontend invoke a smart contract that calls SocialDB

[frol]

Just for reference, @FroVolod extracted the function to estimate deposit for set for bos CLI, and it is here

[evgenykuzyakov]

The UI estimation is probably not precise. State writable view calls on RPC would have addressed that better.