Add documentation of MRuby::Build#disable_cxx_exception .

doc/compile/README.md

@@ -205,6 +205,16 @@ If you need to enable C++ ABI mode explicity add the following:
 
 	conf.enable_cxx_abi
 
+#### C++ exception disabling.
+
+If you need to force C++ exception disable
+(For example using a compiler option to disable C++ exception)
+add following:
+
+	conf.disable_cxx_exception
+
+Note that it must be called before ```enable_cxx_abi``` or ```gem``` method.
+
 ### Debugging mode
 
 To enable debugging mode add the following: