ref-manual: Updated note in the "CentOS Packages" section.

We want to encourage installation of the buildtools tarball for
getting the most up-to-date packages on this build host.

(From yocto-docs rev: 92dbc6e90dffaefc4a91bab81532d24de0d631cc)

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

documentation/ref-manual/introduction.xml

@@ -285,11 +285,17 @@
             <para>
                 The following list shows the required packages by function
                 given a supported CentOS Linux distribution:
-                <note>Depending on the CentOS version you are using, other requirements
-                    and dependencies might exist.
-                    For details, you should look at the CentOS sections on the
-                    <ulink url='https://wiki.yoctoproject.org/wiki/Poky/GettingStarted/Dependencies'>Poky/GettingStarted/Dependencies</ulink>
-                    wiki page.
+                <note>
+                    For CentOS 6.x, some of the versions of the components
+                    provided by the distribution are too old (e.g. Git, Python,
+                    and tar).
+                    It is recommended that you install the buildtools in order
+                    to provide versions that will work with the OpenEmbedded
+                    build system.
+                    For information on how to install the buildtools tarball,
+                    see the
+                    "<link linkend='required-git-tar-and-python-versions'>Required Git, Tar, and Python Versions</link>"
+                    section.
                 </note>
                 <itemizedlist>
                     <listitem><para><emphasis>Essentials:</emphasis>