Update README with key concepts

Author: ilijastuden

README.md

@@ -2,7 +2,14 @@
 
 [![Build Status](https://travis-ci.org/activecollab/databaseobject.svg?branch=master)](https://travis-ci.org/activecollab/databaseobject)
 
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+DatabaseObject library is a set of classes that make work with database just a bit easier. Here are the key concepts:
+
+1. Types - entities that map to a single table,
+2. Objects - type instances that map to a single row of a type table,
+3. Collections - a group of objects that meet the given criteria. This group is perfect for HTTP responses because collections support data tagging and tag validation,
+4. Pool - manage registered types and make multi-type interaction possible,
+6. Producers - customise the way pool produces new instances,
+7. Validators - validate object properties before saving them to the database.
 
 ## To Do