[damned-lies] yelp-xsl is also required to build docs

[Claude Paroz <claudep src gnome org>]

commit fdc7860ce8d2a1c15caf8b0dc4bdaf01673a4d29
Author: Claude Paroz <claude 2xlibre net>
Date:   Mon Sep 17 10:54:09 2018 +0200

    yelp-xsl is also required to build docs

 README | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)
---
diff --git a/README b/README
index a9e88ced..485f3125 100644
--- a/README
+++ b/README
@@ -15,7 +15,8 @@ Requirements
     Markdown (python-markdown) for Team presentation markup rendering.
     translate-toolkit >= 2.2
 
-3 - gettext, intltool, gnome-doc-utils (for stats generation), itstool, yelp-tools
+3 - gettext, intltool, gnome-doc-utils (for stats generation), itstool,
+    yelp-tools, yelp-xsl
 
 4 - [Optional] Django Debug Toolbar
     git clone git://github.com/jazzband/django-debug-toolbar.git