add some documentation on how to use it

CMake/mymachine_openjpeg.cmake

@@ -1,4 +1,7 @@
 cmake_minimum_required(VERSION 2.8)
+# simply copy this file anywhere on your system and execute like this:
+# ctest -S mymachine_openjpeg.cmake -V
+# This will retrieve/compile/run tests/upload to cdash OpenJPEG
 
 # Begin User inputs:
 set( CTEST_SITE             "mymachine" ) # generally the output of hostname