update doc for network setup

Add notice about the Capabilities section before setting up a channel

Change-Id: I0b4d4f85e0465d94bb06fb8f8177b157be95682d
Signed-off-by: zhaochy <zhaochy_2015@hotmail.com>

docs/tutorials/app-dev-env-setup.md

@@ -128,6 +128,9 @@ to use the `configtxgen` tool to generate a `genesis.block`.
 The output, a genesis block file for the orderer, will be used in
 the next step to launch the orderer node.
 
+> Notice some features are non-backwards compatible. To enable features from a particular fabric version,
+the `Capabilities` section in `configtx.yaml` should be updated before generate the `genesis.block`.
+
 ## Start the network
 
 Now we are ready to put it all together. The easiest way to launch the