README: add docker RPC access docs (ethereum#15362)

README.md

@@ -130,6 +130,8 @@ docker run -d --name ethereum-node -v /Users/alice/ethereum:/root \
 
 This will start geth in fast sync mode with a DB memory allowance of 512MB just as the above command does.  It will also create a persistent volume in your home directory for saving your blockchain as well as map the default ports. There is also an `alpine` tag available for a slim version of the image.
 
+Do not forget `--rpcaddr 0.0.0.0`, if you want to access RPC from other containers and/or hosts. By default, `geth` binds to the local interface and RPC endpoints is not accessible from the outside.
+
 ### Programatically interfacing Geth nodes
 
 As a developer, sooner rather than later you'll want to start interacting with Geth and the Ethereum