docs: Update README

[WanLinLin]

Add the Node version 8.0.0 prerequisite.

[kamilogorek]

Hey, I'm happy to add this note, however, I'd need you to ask two things if I may.

a) Move it to more appropriate place - https://github.com/getsentry/raven-node/blob/master/docs/usage.rst#contextwrap at the very bottom of context/wrap section

b) Domains are supported since 0.x. Domains for promises are what's supported since 8.x and they are only used to enhance the context for the current domain. If they're not supported, it'll just be skipped and will use globally set context data. What it means, is that context for regular context/wrap and instrumentation is working prior to 8.x just fine. It's just about the promises.

[WanLinLin]

Sorry, I didn't express it well. Updated usage.rst is committed.

[kamilogorek]

Slightly reworded and merged manually. Thanks!