Add information about signed numbers

[raulpy271]

I was confused while reading the description of the opcode JR NZ, s8. Which is:

If the Z flag is 0, jump s8 steps from the current address stored in the program counter (PC). If not, the instruction following the current JP instruction is executed (as usual).

The description should say that the s8 argument is a signed number, which are encoded in two's complement. This is a usefull information that is missing in the description.

Do you agree?

Besides, this table in helping me a lot in my journey creating a emulator, thanks.