Update CMake and build documentation

docs/tutorial.md

@@ -14,6 +14,9 @@
 
 The simplest way to get Catch2 is to download the latest [single header version](https://raw.githubusercontent.com/CatchOrg/Catch2/master/single_include/catch.hpp). The single header is generated by merging a set of individual headers but it is still just normal source code in a header file.
 
+Alternative ways of getting Catch2 include using your system package
+manager, or installing it using its CMake package.
+
 The full source for Catch2, including test projects, documentation, and other things, is hosted on GitHub. [http://catch-lib.net](http://catch-lib.net) will redirect you there.
 
 
@@ -23,6 +26,9 @@ Catch2 is header only. All you need to do is drop the file somewhere reachable f
 
 The rest of this tutorial will assume that the Catch2 single-include header (or the include folder) is available unqualified - but you may need to prefix it with a folder name if necessary.
 
+_If you have installed Catch2 from system package manager, or CMake
+package, you need to include the header as `#include <catch2/catch.hpp>`_
+
 ## Writing tests
 
 Let's start with a really simple example ([code](../examples/010-TestCase.cpp)). Say you have written a function to calculate factorials and now you want to test it (let's leave aside TDD for now).