feat(12): add note about Diaconescu's theorem
feat(12): add note about Diaconescu's theorem

fpvandoorn/tutorial,avigad/tutorial,avigad/tutorial,soonhokong/lean-tutorial,fpvandoorn/tutorial,Hodapp87/cs6460_project,cjmazey/fork_leanprover_tutorial,fpvandoorn/tutorial,leanprover/tutorial,cjmazey/fork_leanprover_tutorial,Hodapp87/cs6460_project,leanprover/tutorial,cjmazey/fork_leanprover_tutorial,Hodapp87/cs6460_project,soonhokong/lean-tutorial,avigad/tutorial,leanprover/tutorial,soonhokong/lean-tutorial