[gtk-doc] README: mention dblatex dependecy for pdf generation

[Stefan Sauer <stefkost src gnome org>]

commit 4ab76c0ad4ad1f2bf456963301fb3fe2a6c9551c
Author: Stefan Sauer <ensonic users sf net>
Date:   Fri Aug 16 10:53:27 2019 +0200

    README: mention dblatex dependecy for pdf generation

 README | 5 +++++
 1 file changed, 5 insertions(+)
---
diff --git a/README b/README
index 00d10f3..0a0b031 100644
--- a/README
+++ b/README
@@ -42,6 +42,11 @@ The DocBook XSL Stylesheets.
 libxslt & libxml2 >= 2.3.6.
     http://xmlsoft.org/
 
+For PDF output:
+
+the dblatex tool.
+    http://dblatex.sourceforge.net/
+
 Most distributions now have packages for all of these, so I would strongly
 advise that you grab those.