Update KeyPad.md

* Added Caveats section
* Moved warning about multiple keys there
* Added note in caveats section that the module sets watches on columns (in light of STMF103 limit on watches on pins with same numbers) - I had been tearing my hair out a while back with some keys sometimes stopping working, and then it fixed itself when I moved some things around. Much later I realized the watch set by the DHT was clobbering one of the column watches. 
* Added example for the 4x5 keypads available on Ebay and a link to a picture of them.