Add `mcmod.info` to gettingstarted/structuring.md (#56)
Add `mcmod.info` to gettingstarted/structuring.md (#56)

* Add `mcmod.info` to gettingstarted/structuring.md

Define what it is, what it does, and what it can define.

* Touch up the `mcmod.info` section

Clarify some things, and copy the BuildCraft `mcmod.info` into its own
gist.

* Fix a couple of kinks in the wording

Remove repetition from the first sentence and realize that "they 3 do" is
not valid English.

tterrag1098/Documentation,MinecraftForge/Documentation,PaleoCrafter/Documentation