Add bmc support info

[schaarj]

While this repo was very useful, it took me a while to figure out why adding BMC was not as straightforward as expected.

The steps I took trying to add BMC:

• Adding freeipmi to dnf install command in the Dockerfile
• remove "bmc" from bundle --without ...
• Add a bmc.yml file to config/settings.d/

This resulted in the following error during startup
Errors detected on startup, see log for details. Exiting: cannot load such file -- rubyipmi
even though the rubyipmi gem was installed.

The fix was simply changing the command to bundle exec /usr/src/app/bin/smart-proxy in the docker-compose file. So it might be worth changing the command or mentioning the issue in "known issues"