Update Sailor README to include more relevant information for developers

[hannah-bulmer]

A subset of updates that would be useful to make to Sailor README - there are probably more on top of this.

Emitter

Add a section on the Emitter, which functions it is currently available from, what each of the options does, what is supported, where/how they should be used, with what parameters, etc.

The options we currently have defined for emittter are

• this.emit('data', ...)
• this.emit('error', ...)
• this.emit('rebound', ...)
• this.emit('snapshot', ...)
• this.emit('updateSnapshot', ...)
• this.emit('updateKeys', ...)
• this.emit('httpReply', ...) <- not currently used anywhere on platform
• this.emit('end')

Environment Variables

Add a section on list of available environment variables, their purposes.
Add instructions on how to set env variables on the platform.

Hooks

The README already has a section on the startup, shutdown, init hooks - add a section perhaps for all the other functions Sailor looks for? e.g. verifyCredentials, process, getMetaModel, Select Views.
Also make sure the information in this section is still up to date and relevant/correct.

General

• add more examples throughout
• update existing code examples to reflect 2019 syntax
• add known limitations section
  • e.g. if snapshot or emitted keys contains a Date object, this Date object is converted to an ISO string in future executions
• consider publishing this page to the docs for relevant information on interacting with Sailor for developers to be easily accessible