[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
diff --git a/llvm/docs/index.rst b/llvm/docs/index.rst
index 6831f9e..6f4eeec 100644
--- a/llvm/docs/index.rst
+++ b/llvm/docs/index.rst
@@ -95,6 +95,7 @@
    ReportingGuide
    Benchmarking
    Docker
+   BuildingADistribution
 
 :doc:`GettingStarted`
    Discusses how to get up and running quickly with the LLVM infrastructure.
@@ -177,6 +178,10 @@
 :doc:`Docker`
    A reference for using Dockerfiles provided with LLVM.
 
+:doc:`BuildingADistribution`
+  A best-practices guide for using LLVM's CMake build system to package and
+  distribute LLVM-based tools.
+
 
 Programming Documentation
 =========================