README: Use uipc in PubSub example instead of ipc

README: Use uipc in PubSub example instead of ipc

The readme uses the ipc module to explain how to implement simple
pubsub-based messaging using toolbox. However, the ipc module is
somewhat awkward to configure because it uses GPG and requires the
user to generate unencrypted keys.

This commit modifies the readme to introduce the uipc module instead
of the ipc module, since the former is easier to set up.