ref-manual: Updated the GCC 5 migration section for 2.0

Added another link to Josh's porting guide.

(From yocto-docs rev: a1860811628aeddd579f92844f3e6bfcbded2936)

Signed-off-by: Scott Rifenbark <srifenbark@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/ref-manual/migration.xml

@@ -2343,8 +2343,10 @@
         </para>
 
         <para>
-            For further details, please see
-            <ulink url='https://gcc.gnu.org/gcc-5/changes.html'></ulink>.
+            For further details, see
+            <ulink url='https://gcc.gnu.org/gcc-5/changes.html'></ulink> and
+            the porting guide at
+            <ulink url='https://gcc.gnu.org/gcc-5/porting_to.html'></ulink>.
         </para>
 
         <para>