Add chapter concepts to readme.md

readme.md

@@ -15,6 +15,11 @@ The stellar components are:
 
 ![stellar-high-level-design](./docs/imgs/stellar-high-level-design.svg)
 
+## Concepts
+Session is an abstract of Layer 4-7 connection, i.e., TCP Session, HTTP Session, DNS Session. A session can derive child sessions.
+Session Extra Data is a plugin defined data structure that attached to a session, it's lifetime is same as the session.
+Event is something happened in a session, i.e., TCP OPENING, HTTP Request, Session Extra Data updates.
+
 ## Packet IO Library
 ```
 struct stellar_packet;