Add a CONTRIBUTING document

Copied from Chef, updated a bit for Ohai and recent changes
fail when more than one collect_data blocks defined per platform
add PluginDefinitionError to Ohai, rescue in loader.rb
add exceptions to ohai/exception.rb
check for detailed warning message
fix mime-types dependency version
Merge pull request #217 from opscode/OC-10247

Fail Ohai when > 1 collect_data blocks defined per platform