adds docs for users without http access

docs/usage.md

@@ -13,6 +13,9 @@ In this chapter there is some general information for people (e.g. teachers/cour
     - `/group_work`: read and write enabled for all participants, and shared between all containers. This can be used to share data/scripts between students. This directory can be backed up. 
     - `~/workdir`: read and write enabled, and only shared between containers assigned to the same participant. This directory can be backed up and shared as a tarball at the end of the course. 
 - All directories other than the shared directories only exist within the container. 
+- Some firewalls do not support http connections. In that case, use port forwarding instead of using the public IP address. This would require an extra user to set up the port forwarding:
+    - Create a new user on the EC2 instance with public and private keys
+    - Set up port forwarding on the local machine with `ssh -L 10034:public_ip_address:10034 -i /path/to/private_key.pem user@public_ip_address`
 
 ## Jupyter containers