[Docs] Add some requirements to the Testing Guide

Patch by Diana Picus.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@269566 91177308-0d34-0410-b5e6-96231b3b80d8

docs/TestingGuide.rst

@@ -25,6 +25,10 @@ In order to use the LLVM testing infrastructure, you will need all of the
 software required to build LLVM, as well as `Python <http://python.org>`_ 2.7 or
 later.
 
+If you intend to run the :ref:`test-suite <test-suite-overview>`, you will also
+need a development version of zlib (zlib1g-dev is known to work on several Linux
+distributions).
+
 LLVM testing infrastructure organization
 ========================================
 
@@ -535,6 +539,8 @@ the last RUN: line. This has two side effects:
 (b) it speeds things up for really big test cases by avoiding
     interpretation of the remainder of the file.
 
+.. _test-suite-overview:
+
 ``test-suite`` Overview
 =======================