Add a hint about use with monorepos

[trusktr]

The README lacks a mention of the keyword "monorepo". The word "sub projects" could be good too.

FWIW, I found all of these projects before finally stumbling on Builder:

• github.com/Aldlevine/comptroller
• github.com/lerna/lerna
• github.com/boltpkg/bolt
• github.com/guigrpa/oao
• github.com/Microsoft/web-build-tools/blob/master/apps/rush
• github.com/northbrookjs/northbrook
• github.com/studio-b12/monopod
• github.com/sompylasar/zmey-gorynych
• github.com/asini/asini
• github.com/sencha/mondorepo
• github.com/signavio/binge
• github.com/jdiamond/monotool
• npmjs.com/package/redhot

Using some keywords like "monorepo", "subprojects", "multi-package repositories" would help make Builder more visible.

---

I'm actually still wondering how I will use this for a monorepo, but I bet the result will be nice. I may have spoken to soon about adding "monorepo" somewhere in README, since I haven't actually tried it yet. Hoping it'll work though!

[coveralls]

Coverage remained the same at 93.664% when pulling 0444a4d on trusktr:patch-1 into 390eaa4 on FormidableLabs:master.