add lines on the necessity of known hosts

README.md

-
 # How to work with Jupyter everywhere
 
 
@@ -146,6 +145,11 @@ And run it to connect to `host.example.com`:
 ./run_chromium_through_ssh_tunnel.sh user@host.example.com
 ```
 
+Note that these steps will only work, if `host.example.com` is a known host in 
+`.ssh/known_hosts`. (To test this, simply create a ssh connection to 
+`host.example.com`, if it is not yet a known host, you will be asked whether to 
+continue and if you do, `host.example.com` will be added to `.ssh/known_hosts`.)
+
 
 ### Connect your browser to the remote Jupyterlab