Document new "Pay for service" system
The new "Pay for service" system is one of the most interesting features of 0.15, i think would be useful to have some documentation about it. Could be a dedicated dev guide, and addition to the existent dev guides or something else (ence the generic 'docs' label), but IMHO, the feature should be well documented.
More info: https://github.com/monero-project/monero/pull/5357/commits/2899379791b7542e4eb920b5d9d58cf232806937
Feel free to propose how to better add it.