ref-manual: Added note about installing Git-Python package

buildhistory-diff tool requires the Git-python package.
I added a note indicating this.

Fixes [YOCTO #9712]

(From yocto-docs rev: 61814503f5656b241646d43c208c6bcaf530a282)

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

documentation/ref-manual/usingpoky.xml

@@ -951,6 +951,19 @@
         * PR changed from "r0" to "r1"
         * PV changed from "0.1.10" to "0.1.12"
                 </literallayout>
+                <note>
+                    The <filename>buildhistory-diff</filename> tool requires
+                    the <filename>GitPython</filename> package.
+                    Be sure to install it using Pip3 as follows:
+                    <literallayout class='monospaced'>
+   $ pip3 install GitPython --user
+                    </literallayout>
+                    Alternatively, you can install
+                    <filename>python3-3</filename> using the appropriate
+                    distribution package manager (e.g.
+                    <filename>apt-get</filename>, <filename>dnf</filename>, or
+                    <filename>zipper</filename>).
+                </note>
             </para>
 
             <para>