Add migration documentation

[EECOLOR]

This might help to get started:

require('firebase-queue') -> require('@kaliber/firebase-queue')

remove spec related database logic

const specCreated = specsRef.child(specId).set({
    in_progress_state: 'in_progress',
    finished_state: 'finished',
    error_state: 'error',
    timeout: TIMEOUT,
    retries: 0
  }).catch(reportError)

change queue instantiation:

• it's a single object argument
• spec is passed as option, don't forget to rename spec properties
• numWorkers is 1 by default
• timeout and retries are removed (see motivation)
• rename task handler to processTask
• add reportError

const options = { specId, numWorkers: 1 }
new Queue({ tasksRef, specsRef }, options, handleRequest)

const spec = {
    inProgressState: 'in_progress',
    finishedState: 'finished',
    errorState: 'error',
}
new Queue({ tasksRef, options: { spec }, processTask, reportError })

adjust process task function

• adjust signature
• return promise (if relevant)
• remove calls to resolve and reject

function processTask(data, progress, resolve, reject) -> function processTask(data, { setProgress })