Add helper support for MMS

[grokify]

MMS is now supported in the API. This is similar to Fax support.

• https://developer.ringcentral.com/api-docs/latest/index.html#!#RefCreateSMSMessage

[kirill-konshin]

Only documentation has to be updated, the process is 100% the same as for fax: https://github.com/ringcentral/ringcentral-js/blob/master/README.md#fax