Recommend using ninja in the README.

Author: thewtex

README.md

@@ -13,8 +13,13 @@ example:
     git clone https://github.com/CastXML/CastXMLSuperbuild.git
     mkdir CastXMLSuperbuild-build
     cd CastXMLSuperbuild-build
-    cmake ../CastXMLSuperbuild
-    make
+    cmake -GNinja ../CastXMLSuperbuild
+    ninja
+
+This requires [ninja](https://ninja-build.org/), which comes with recent
+Visual Studio on Windows, can be installed on macOS with `brew install ninja`,
+and can be installed with the `ninja-build` package in most Linux package
+managers.
 
 The resulting [castxml]{.title-ref} executable will be located at
 [CastXMLSuperbuild-build/castxml/bin/castxml]{.title-ref}.