documentation/README: how to upgrade Sphinx packages

(From yocto-docs rev: 199c6ef4b64d6c7f672ac183d3bf73c8ff3bb2c7)

Signed-off-by: Michael Opdenacker <michael.opdenacker@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/README

@@ -113,13 +113,18 @@ documentation with Python3.
 Sphinx might be available in your Linux distro packages repositories,
 however it is not recommended to use distro packages, as they might be
 old versions, especially if you are using an LTS version of your
-distro. The recommended method to install Sphinx and all required
-dependencies is to use the Python Package Index (pip).
+distro. The recommended method to install the latest versions of Sphinx
+and of its required dependencies is to use the Python Package Index (pip).
 
 To install all required packages run:
 
  $ pip3 install sphinx sphinx_rtd_theme pyyaml
 
+To make sure you always have the latest versions of such packages, you
+should regularly run the same command with an added "--upgrade" option:
+
+ $ pip3 install --upgrade sphinx sphinx_rtd_theme pyyaml
+
 Also install the "inkscape" package from your distribution.
 Inkscape is need to convert SVG graphics to PNG (for EPUB
 export) and to PDF (for PDF export).