[Docs][Phabricator] Mention how to create a draft revision

https://llvm.org/docs/Phabricator.html have two links to Arcnist guide but
none of them mention how to create a draft revision. It would create some less noise if
developers create draft revisoin in this(--draft) way instead of [WIP] tag way.

Reviewed By: dblaikie

Differential Revision: https://reviews.llvm.org/D108970