[docs] Add new document on building distributions

Summary:
This document is an attempt to provide a guide for best practices for using the LLVM build system to generate distributable LLVM-based tools.

Most of the document is geared toward distributions of LLVM-based toolchains, but much of it also applies to distributing other LLVM-based tools and libraries.

Reviewers: tstellar, phosek, jroelofs, hans, sylvestre.ledru

Reviewed By: tstellar

Subscribers: smeenai, dschuff, arphaman, winksaville, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D62040

llvm-svn: 361272