[librsvg/librsvg-2.44] Update compiling docs for debian

[Federico Mena Quintero <federico src gnome org>]

commit 8599a988b929de8a915ab845c7156ce1c02ac70f
Author: Jordan Petridis <jpetridis gnome org>
Date:   Tue Feb 5 19:41:23 2019 +0000

    Update compiling docs for debian
    
    Close #411

 COMPILING.md | 6 ++++++
 1 file changed, 6 insertions(+)
---
diff --git a/COMPILING.md b/COMPILING.md
index 690da92b..e4853507 100644
--- a/COMPILING.md
+++ b/COMPILING.md
@@ -68,6 +68,12 @@ gtk-doc-tools git libgtk-3-dev \
 libxml2-dev libcroco3-dev libcairo2-dev libpango1.0-dev
 ```
 
+Additionally, as of September 2018 you need to add `gdk-pixbufl` utilities to your path, see #331 for more.
+
+```sh
+PATH="$PATH:/usr/lib/x86_64-linux-gnu/gdk-pixbuf-2.0"
+```
+
 ### Fedora based systems
 
 ```sh