Add more great ideas to the VM spec

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

vm.md

@@ -41,6 +41,11 @@ CPU flags are addressed by bit index, going from right to left.
     * Overwriting a register without its value being used
     * Mixing arithmetic with bit twiddling on the same target
 
+## Register ideas
+
+* NULL - a register that will always be zero for reading and will not change after writing.
+    * Other possible names: Z, NIL
+
 # Instructions
 
 ## Arithmetic
@@ -48,7 +53,6 @@ CPU flags are addressed by bit index, going from right to left.
 Arithmetic instructions store their result in the first register specified. Overflow is handled by
 wrapping around to 0.
 
-
 * Add
     * **Params**: REG1, REG2
     * `REG1 = REG1 + REG2`