Add comparison of sugar to README.md

[indexzero]

@3rd-Eden been poking around at the internals of bigpipe to get my bearings and for predefine I see the value, but I think it's fair to call it "sugar". I love sugar, but it's often hard to sell to people.

Suggesting here a couple of examples that show how the new API predefine provides desugars to plain Javascript. It will make the use-case more clear.

Related: any other base/core libraries that should be on my radar as I dig into BigPipe? Currently looking at:

• pagelet
• routable
• temper
• fusing
• pipe.js