[devhelp] readme: add reference to docs/professional-services.md file

[Sébastien Wilmet <swilmet src gnome org>]

commit 0691d2c1dc555f27a32bd671dd7bb12fcc20c3cb
Author: Sébastien Wilmet <swilmet gnome org>
Date:   Mon Nov 4 09:25:45 2019 +0100

    readme: add reference to docs/professional-services.md file

 README | 5 +++++
 1 file changed, 5 insertions(+)
---
diff --git a/README b/README
index a243208a..b2ae1596 100644
--- a/README
+++ b/README
@@ -5,6 +5,11 @@ The Devhelp web page:
 
     https://wiki.gnome.org/Apps/Devhelp
 
+Professional services
+---------------------
+
+See the file: docs/professional-services.md
+
 How to contribute
 -----------------