add mistake report: just two registers

MISTAKES.md

@@ -27,3 +27,10 @@ move CPU to an infinite loop.
 
 It is probably easier to implement than writing proper
 lexer/parser, but error reporting can be weird.
+
+## Two registers are really not enough
+
+Just imagine, you are trying to write a loop, you use
+`R1` as your index variable and `R0` for your loop condition.
+Now you have already ran out of registers and you didn't even
+start writing loop body!