Added tested systems to README

README

@@ -55,9 +55,13 @@ GTSAM requires the following libraries to be installed on your system:
  - Cmake version 2.6 or higher
 
 Tested compilers
- - gcc
- - clang
- - OSX gcc
+ - GCC 4.2-4.7
+ - Clang 2.9-3.2
+ - OSX GCC 4.2
+ 
+Tested systems:
+ - Ubuntu 11.04, 11.10, 12.04
+ - MacOS 10.6, 10.7
 
 2)
 GTSAM makes extensive use of debug assertions, and we highly recommend you work