abci: clarify what abci stands for (#336)

* abci: clarify what abci stands for

* link to abci type protos.

spec/abci/README.md

@@ -7,14 +7,15 @@ parent:
 
 # ABCI
 
+ABCI stands for "**A**pplication **B**lock**c**hain **I**nterface".
 ABCI is the interface between Tendermint (a state-machine replication engine)
 and your application (the actual state machine). It consists of a set of
-_methods_, where each method has a corresponding `Request` and `Response`
-message type. Tendermint calls the ABCI methods on the ABCI application by sending the `Request*`
-messages and receiving the `Response*` messages in return.
+_methods_, each with a corresponding `Request` and `Response`message type. 
+To perform state-machine replication, Tendermint calls the ABCI methods on the 
+ABCI application by sending the `Request*` messages and receiving the `Response*` messages in return.
 
-All message types are defined in a [protobuf file](https://github.com/tendermint/tendermint/blob/master/proto/tendermint/abci/types.proto).
-This allows Tendermint to run applications written in any programming language.
+All ABCI messages and methods are defined in [protocol buffers](https://github.com/tendermint/spec/blob/master/proto/abci/types.proto). 
+This allows Tendermint to run with applications written in many programming languages.
 
 This specification is split as follows: