Document Executor and its modification

[6r1d]

As far as I understand, the Executor is useful for access control.
We need to explain how it can be modified for that purpose.

As pointed by @Mingela, one should look for the list of the default permissions in https://github.com/hyperledger/iroha/blob/7523a69475a42ac4b2c0b1893b8bbd01c85331f4/smart_contract/executor/src/default/tokens.rs#L66

[nxsaken]

Create issues for guide/reference/examples