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