Adds meshms markdown documentation

Problem terminology "Your keyring".  The keyring does not belong to the HTTP client, so "your" is not appropriate.  Also want consistency, use either `<sid1>` or `SID1` but not both.  The clause "When sending this message," is unnecessary, it is implied in the context.  Suggest:

    `<sid1>` must be an identity that is currently open (unlocked) by the Serval DNA server.