feat(02): describe 'let'
feat(02): describe 'let'

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