Update docs: read() required before readable event

[wanderview]

This is one possible fix for #4695.

[indutny]

@isaacs is it really the way it should be documented? Feels really odd to me.

[wanderview]

I agree its a bit clunky, but that seems how it operates now. @TooTallNate indicated in #4695 that there is a performance issue with an implicit _read() when the stream is constructed.

Perhaps we could default to the more intuitive behavior where the first readable event does not require a read(), but provide a tunable constructor option to disable this behavior. Something like:

var r = new Readable({explicitRead: true});

[indutny]

I guess there is another more complex design problem behind this. If you'll miss 'readable' event - you won't know that it has happened. And this is a situation that was always happening with streams before streams2, so you should explicitly start reading from stream to get first readable event.

[indutny]

And, considering this, this pull request LGTM.

[wanderview]

As of v0.9.11, my original test no longer fails. At least for streams that has push() called asynchronously, you no longer need to call read(0) to kick them off. Closing since these doc changes are no longer relevant.