message: Adds mcmaster carr to source key list
diff: @@ -14,6 +14,7 @@ For an upgrade, check the [Pizza build](../pizza_build.md).
  ### Purchasing Source Key
  * **Z** = Amazon
  +* **C** = McMaster-Carr4!@(4_Prototype_Assembly/Alpha_Build_Instructions) -
  * **A** = Ali Express
  * **M** = Mouser
  * **O** = Monoprice
  