Add docs on reducing bundle size for Ramda.

[AndrewSouthpaw]

Based on discussion in several threads, including:

• Profit from Tree Shaking in Ramda in Webpack #2355
• Make ramda more modular #1505
• this repo comparing tree shaking

I figured it'd be a good idea to actually add some of this collective knowledge to the docs, since most people probably don't realize their bundle size may be bigger than they'd expect.

[CrossEye]

This looks great to me. Thanks.

Anyone else have suggestions? @ramda/core ?

[JasonSooter]

Does a distinction between using src and es directories need to be made?

[CrossEye]

@JasonSooter: that's probably a good idea. Any specific suggestions?

[JasonSooter]

@CrossEye Well, I don't really have any specific suggestions except maybe just adding that distinction in the cherrypicking section. I've been using the es directory because I have come to think that is the recommended way. And it seems to work fine for me. It also has stayed true to keeping bundle size to only imported functions after I setup module concatenation with Webpack. The only downside I have come across and that I have yet to dig into is that Flow cannot find types related to imported functions like it can if using import * as R from 'ramda' Let me know if I can be of any assistance.

[CrossEye]

Looks great!

[CrossEye]

I'm going to merge as it is. If someone has a good suggestion for changes to document the use of es vs src, please open another PR.