Tagging: Create a draft of a proposal to refine power plant tagging

[flacombe]

Power plants tagging in OS has been set in 2013 and need refinement

The proposal remain to be drafted, with following points in mind (feel free to complete):

• Refine fuels versus technologies in plant:method, generator:method and generator:type
• Include captive power plants in the definitions as well (why not merging with generator:plant key)

This work shouldn't be confused with documentation process of existing and sustainable practices, that will be addressed in #10, #11 and #12

[Ly0n]

This is also #17