I was wondering. If I own a patent on something, can someone sell a "kit" that includes everything you need to make the thing I have patented with instructions on how to make it.
It depends. 35 USC 271(c) provides that:
"Whoever offers to sell or sells within the United States or imports into the United States a component of a patented machine, manufacture, combination, or composition, or a material or apparatus for use in practicing a patented process, constituting a material part of the invention, knowing the same to be especially made or especially adapted for use in an infringement of such patent, and not a staple article or commodity of commerce suitable for substantial noninfringing use, shall be liable as a contributory infringer."
So, if the kit contains more than just "staple articles or commodities of commerce" -- i.e., more than just generic, off the shelf parts -- the kit seller can be liable for contributory infringement.
The problem with contributory infringement is that you need to have direct infringement first. Here, what you would need is someone who bought the kit, then built and\or practiced your invention. That person who bought the kit would then be directly infringing on your patent -- you could sue them for infringement, then join the makers of the kit and sue them for contributory infringement.
But, just offering to sell the kit alone -- as long as nobody buys it -- would not be infringement. However, as soon as someone bought the kit, there would be infringement, and the kit maker would then become liable for contributory infringement.
Does the law only say that someone can't manufacture and sell something patented?
In addition to the above, 35 USC 271(a) states:
"Except as otherwise provided in this title, whoever without authority makes, uses, offers to sell, or sells any patented invention, within the United States, or imports into the United States any patented invention during the term of the patent therefor, infringes the patent."
Or does it also include the instructions on producing the item patented?
In principle, the patent itself -- which is a public record -- is supposed to include enough information so that one "skilled in the art" would be able to practice your invention without "undue experimentation." Selling the instructions themselves should, in principle at least, be no more infringing than just handing out copies of the patent. So its not the instructions that are infringing -- its the parts in the kit that are infringing.