docs/manual: document python-aiohttp needed for pkg-stats

Signed-off-by: Raphaël Mélotte <raphael.melotte@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
(cherry picked from commit 095bd205ae22c285bab272ad341948236dac5bcf)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

docs/manual/prerequisite.adoc

@@ -96,3 +96,6 @@ corresponding tool on the host system:
 * Graph generation tools:
 ** +graphviz+ to use 'graph-depends' and '<pkg>-graph-depends'
 ** +python-matplotlib+ to use 'graph-build'
+
+* Package statistics tools ('pkg-stats'):
+** +python-aiohttp+