Add primitive domains and criteria feature

docs/primitive-domains.md

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+# fflib-apex-extensions
+
+## Primitive Domains
+
+A huge benefit of using primitive domains, is that you write less code. 
+Instead of:
+```apex
+List<String> myStrings = getSomeStrings();
+```
+You can write:
+```apex
+Strings myStrings = getSomeString();
+```
+
+It is a small thing, but it makes things looks much nicer.
+
+
+One other benefit is that you can encapsulate more logic as Lists. You can see these lists as a Domain.
+Take a look at the SObjects primitive domain, it is the new source for extending domain classes.
+
+
+