Document naming convention #280 in CONTRIBUTING.md

[akinomyoga]

[...] the variables for plugins are supposed to have the name of OMB_PLUGIN_{PluginName}_{ConfigName} (see the xterm and fasd plugins for examples). We do not include the variables for specific plugins in the bashrc template by default, but instead, describe the usage in the README of each plugin to let the user add it in their bashrc by themselves.

This is valuable information that can be added to CONTRIBUTING.md!

Originally posted by @sanzoghenzo in #371 (comment)

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This is valuable information that can be added to CONTRIBUTING.md!

Thank you for the suggestion! Actually, the convention of OMB_THEME_{plugin}_{config} is a part of a proposal in Discussion #280. There seem to be no further opinions on the proposal, so I guess we can now freeze the convention and put it somewhere in the documents. The current content of CONTRIBUTING.md is the one inherited from the upstream oh-my-zsh. I actually think that we need to do an overhaul of CONTRIBUTING.md at some point, but incremental fixes are also welcome. By the way, there is also a PR on CONTRIBUTING.md at #90, but I don't see reasons for large changes in #90 so currently am leaving it as is.

Originally posted by @akinomyoga in #371 (comment)