core/settings: add README

Add simple explanation of how the program flow is.

Signed-off-by: Jan Iversen <jani@apache.org>

core/settings/README.md

@@ -0,0 +1,91 @@
+# Short explanation of how qPref works
+
+## System startup:
+
+### core/qt-init.cpp init_qt_late()
+does
+```
+qPref::load();
+```
+to load all properties from all qPref* classes
+
+## System shutdown:
+
+### subsurface-mobile-main.cpp and subsurface-desktop-main.cpp main()
+calls
+```
+call qPref::sync()
+```
+to save all changes to disk, this is needed because part of the program
+modifies struct preferences instead of calling the set method.
+
+#### EXCEPTION:
+Variables not present in struct preferences (local static variables in the class
+are not written, see later.
+
+## System startup/shutdown common handling:
+```
+qPref::load() calls qPref::doSync(false)
+qPref::sync() calls qPref::doSync(true)
+```
+
+### qPrefDoSync()
+qPref::doSync() calls qPref<xyz>::doSync() for all qPref* classes
+Meaning qPref knows which classes exist, but not which variables
+
+### qPref*::doSync() calls
+```
+disk_<variable name>() for each variable
+```
+
+#### EXCEPTION: 
+some variables are not in struct preferences, but local static variables
+which can only be accessed through the set/get functions, therefore there
+are no need to sync them
+```
+	if (!doSync) // meaining load
+		load_<variable_name>()
+```
+
+### qPref*::disk_*()
+qPref*::disk_*() calls qPrefPrivate::propSetValue to save a property, 
+and qPrefPrivate::propValue() to read a property. The function also
+keeps struct preferences in sync.
+
+### qPrefPrivate::propSetValue()
+qPrefPrivate::propSetValue() is static and calls QSettings to write property
+
+### qPrefPrivate::propValue()
+qPrefPrivate::propValue() is static and calls QSettings to read property
+
+### macros 
+the macros are defined in qPrefPrivate.h
+
+the macros are used in qPref*cpp 
+
+## Reading properties
+Reading a property is done through an inline function, and thus no more expensive
+than accessing the variable directly.
+
+```
+qPref*::<variable_name>()
+```
+
+## Setting properties
+Setting a property is done through a dedicated set function which are static to
+simplify the call:
+```
+qPref<class>::set_<variable_name>(value)
+```
+like:
+```
+qPrefDisplay::animation_speed(500);
+```
+
+the set function updates struct preferences (if not a local variable), and saves
+the property to disk, however save is only done if the variable is changed.
+
+##  Updating struct preferences 
+When a program part updates struct preferences directly, changes are NOT saved to disk
+if the programm crashes, otherwise all variables are saved to disk with the program exists.
+