[gnome-system-monitor] Added missing info to HACKING

[Robert Roth <robertroth src gnome org>]

commit f1d5bcc470e12315162c5b7006d603230793bd3d
Author: Robert Roth <robert roth off gmail com>
Date:   Tue Jan 21 12:57:48 2020 +0200

    Added missing info to HACKING

 HACKING | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)
---
diff --git a/HACKING b/HACKING
index 9fceac42..dfa273a5 100644
--- a/HACKING
+++ b/HACKING
@@ -1,12 +1,15 @@
 # Meson
 
-## configure
+## configure creates the build directory
 meson build --prefix /home/benoit/gnome
 
-## build
+## build - compiles the source
 ninja -C build
 
-## clean
+## install - required to be able to run the application
+ninja -C build install
+
+## clean - removes built files to do a clean build
 ninja -C build -t clean
 
 ## start from scratch