Update README to describe grunt build system

[dmethvin]

For someone who just wants to get up and running building jQuery, the grunt docs aren't clear at all. It's yet not a common enough build system to assume that everyone knows how to get and use it.

The grunt project ​README assumes the first thing you want to do is set up your own gruntfile, which isn't the case if you just need it to build jQuery. I didn't see any obvious reference to the npm install grunt that's needed, not even in the "Getting Started" guide!

Ideally we'd be able to delegate at least some of this to the grunt README, it really should be there.

[Rick Waldron]

Confirmed. I definitely need some help with this - but had completely forgotten that it needed to be addressed.

[dmethvin]

Oh, I see npm install grunt is the *last* thing discussed in the README, probably just needs to be up top.

[Rick Waldron]

Right, I didn't want to boot the existing instructions yet, so I threw it in later. Sorry for the completely lack of clarity :D

[dmethvin]

Actually, if jaubourg is going to land JHP that will require more docs so we might as well do them together; sounds like a good topic for tomorrow's meeting. Scott pointed out that npm install does most of the work but I do think we should have a quick-start. That will become more important if we're going to have people building custom versions.

[dmethvin]

Landed here and tweaked later: ​https://github.com/jquery/jquery/commit/7c23b77af2477417205fda9bde5208a81e57e40e#diff-2

Thanks rwaldron!