[gtk-doc] Update requirements.

[Stefan Sauer <stefkost src gnome org>]

commit bfe00660376d81789fe01634098857adcb61c0ed
Author: Stefan Sauer <ensonic users sf net>
Date:   Tue Oct 16 21:18:05 2018 +0200

    Update requirements.
    
    Fixes #55.

 README | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)
---
diff --git a/README b/README
index 1cf00a7..dc0794b 100644
--- a/README
+++ b/README
@@ -25,10 +25,12 @@ dropped.
 Requirements
 ============
 
-Python 2.7, 3.x - the secondary scripts are written in Python
+Python 3.x
     http:///www.python.org
 
-Python-six - for python 2,3 compatibility
+Additional python modules:
+    For the tests: nose
+    For mkhtml2 (experimental): anytree, lxml and pygments
 
 For XML output (recommended):