abci: add ResponseInitChain.app_hash (#140)

spec/abci/abci.md

@@ -243,6 +243,7 @@ via light client.
   - `ConsensusParams (ConsensusParams)`: Initial
     consensus-critical parameters (optional).
   - `Validators ([]ValidatorUpdate)`: Initial validator set (optional).
+  - `AppHash ([]byte)`: Initial application hash.
 - **Usage**:
   - Called once upon genesis.
   - If ResponseInitChain.Validators is empty, the initial validator set will be the RequestInitChain.Validators
@@ -252,6 +253,8 @@ via light client.
     set proposed by tendermint (ie. in the genesis file), or if it wants to use
     a different one (perhaps computed based on some application specific
     information in the genesis file).
+  - The returned `AppHash` must match the hash specified in the genesis file, and will be
+    recorded in the initial genesis block.
 
 ### Query
 

spec/core/data_structures.md

@@ -413,7 +413,7 @@ block.AppHash == state.AppHash
 
 Arbitrary byte array returned by the application after executing and commiting the previous block. It serves as the basis for validating any merkle proofs that comes from the ABCI application and represents the state of the actual application rather than the state of the blockchain itself.
 
-The first block has `block.Header.AppHash == []byte{}`.
+The first block's `block.Header.AppHash` is given by `ResponseInitChain.app_hash`, which must match the app hash specified in the genesis file.
 
 ### LastResultsHash