[librsvg: 2/4] Note in CONTRIBUTING.md about adding properties.

[Federico Mena Quintero <federico src gnome org>]

commit 12d6facc9ca889085812125c8228fbdc983fc1ad
Author: Federico Mena Quintero <federico gnome org>
Date:   Mon Aug 24 19:44:34 2020 -0500

    Note in CONTRIBUTING.md about adding properties.

 CONTRIBUTING.md | 3 +++
 1 file changed, 3 insertions(+)
---
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index a9502e8b..f739a240 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -85,6 +85,9 @@ with a sample SVG file.
 The library's internals are being documented at
 https://gnome.pages.gitlab.gnome.org/librsvg/doc/rsvg_internals/index.html
 
+Please see that documentation for common tasks like adding support for
+a new CSS property.
+
 ### Working on the source
 
 Librsvg uses an autotools setup, which is described in detail [in this