add information how to use GTEST_ROOT

thanks @pinotree for merge request

doc/COMPILE.md

@@ -133,6 +133,13 @@ TARGET_PLUGIN_FOLDER:
 	to other libraries.
 
 
+GTEST_ROOT:
+	Specifies the root of the GoogleTest sources, to be used
+	for some of the tests. A CMakeLists.txt inside GTEST_ROOT
+	will be searched as way to detect a valid GoogleTest source
+	directory.
+	If it is "", an internal version of gtest will be used.
+
 It is recommended that you browse through all of the options.
 Afterwards press c again (maybe multiple times until all variables are
 resolved) and then g to generate.  Finally press e to exit.