check node log (#177)

Author: dustinxie

changelog/v1.2-instruction.md

@@ -27,6 +27,11 @@ docker run -d --restart on-failure --name iotex \
         -config-path=/etc/iotex/config_override.yaml \
         -genesis-path=/etc/iotex/genesis.yaml
 ```
+Verify the node is up and running by:
+```
+docker logs iotex --tail=100
+```
+You should see log output from the node.
 
 **If your node is syncing blocks very slow, see [here](#slow-sync)**
 
@@ -58,6 +63,12 @@ recommend you do the upgrade in an epoch when the node is not chosen as active
 delegate. This would prevent your node from being possibly probated and lose
 productivity.
 
+Verify the node is up and running by:
+```
+docker logs iotex --tail=100
+```
+You should see log output from the node.
+
 **If your node is syncing blocks very slow, see [here](#slow-sync)**
 
 ## Instruction for Testnet v1.2.0 Upgrade