[zsh-completion] lb-run completion

The README file is intended for users, not for maintainers. The README file isn't strictly related to the lb-run completion (discusses the general usage), but the entire customization section is dedicated to it, so it isn't independent either. I didn't do anything to deploy the README (unsure about where to put it, I guess the cleanest would be to actually drop it from here and put the content - once agreed upon - onto the twiki with the LbEnv documentation.