documentation/adt-manual/adt-prepare.xml: Added text about host gcc

It is important that the environment uses a host gcc when running the
adt installer.  I put some text in noting that.

(From yocto-docs rev: a443743133aa6ae1ec647949ad7045e13a3a84e4)

Signed-off-by: Scott Rifenbark <scott.m.rifenbark@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/adt-manual/adt-prepare.xml

@@ -166,7 +166,10 @@
 
             <para>
                 After you have configured the <filename>adt_installer.conf</filename> file, 
-                run the installer using the following command:
+                run the installer using the following command.
+                Be sure that you are not trying to use cross-compilation tools. 
+                When you run the installer, the environment must use a 
+                host <filename>gcc</filename>:
                 <literallayout class='monospaced'>
      $ ./adt_installer
                 </literallayout>