Add notes about google api promises to docs

[motin]

For whatever reason, the pseudo gmail client returned promises, causing havoc in this library.

In the end, I had to change the usage example to resolve promises using .flatMap(_h) before piping to gbs:

.flatMap(_h) // resolve promises
.pipe(this.googleBatchStream.pipeline(1, 5))

and

.flatMap(_h) // resolve promises
.pipe(GBS.pipeline(100, 5)) //Run in batches of 100. Use quota of 5 (for users.messages.get).