[folks] HACKING: Update documentation for strict-non-null mode

[Philip Withnall <pwithnall src gnome org>]

commit 1beab23520bbca100bcf54006ef4958783002413
Author: Philip Withnall <philip tecnocode co uk>
Date:   Mon Mar 11 20:16:02 2013 +0000

    HACKING: Update documentation for strict-non-null mode
    
    Mention how to compile with --enable-experimental-non-null.
    
    See: https://bugzilla.gnome.org/show_bug.cgi?id=695628

 HACKING |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)
---
diff --git a/HACKING b/HACKING
index 0ff14f3..b41bf66 100644
--- a/HACKING
+++ b/HACKING
@@ -111,6 +111,8 @@ Vala-specific rules
     programmer's intentions clearer about whether a variable may be null). The
     ultimate goal is for folks to compile correctly with Vala’s strict-non-null
     mode enabled (https://live.gnome.org/Vala/Tutorial#Strict_Non-Null_Mode).
+    You can compile folks with strict-non-null mode enabled using:
+        make VALAFLAGS=--enable-experimental-non-null
 
 12. Place the (private) member variable declaration for a variable which backs a
     property next to the (public) property declaration, rather than at the top