[gnome-system-monitor] Added description how to install the needed dependencies. I struggled with this for two hours.

[Robert Roth <robertroth src gnome org>]

commit 1270a04572c0c5569bba774acd439282287d365f
Author: Julian Wollersberger <24991778+Julian-Wollersberger users noreply github com>
Date:   Fri Mar 27 19:15:44 2020 +0100

    Added description how to install the needed dependencies.
    I struggled with this for two hours.

 HACKING | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)
---
diff --git a/HACKING b/HACKING
index dfa273a5..13adb319 100644
--- a/HACKING
+++ b/HACKING
@@ -1,3 +1,6 @@
+# Dependencies - can be found in meson.build
+apt-get install meson libglibmm-2.4-dev libgtkmm-3.0-dev libgtop2-dev librsvg2-dev libxml2-dev libsystemd-dev
+
 # Meson
 
 ## configure creates the build directory
@@ -25,4 +28,3 @@ rm -rf build
 
 ## build
 make
-