Fluid 5439: Adding some Prefs Framework files

@cindyli or @jobara:
This pull request adds the following files:

AuxiliarySchemaForPreferencesFramework.md
Builder.md
Enactors.md
PreferencesEditor.md
PrimarySchemaForPreferencesFramework.md

and updates README.md 

can't use the back tick here for code highlighting, you'll need to actually wrap the content in a `<code>` block