Create CONTRIBUTING.md

Updates

## Instruction MOV
MOV is renamed as MVR and MOV, DEC, and INC become pseudos using MVR

### Before:
|`MOV`| `D, S`          | `XXXXXXXXSSDD0000` | Move value at source register to destination register|

### After:
|`MVR`| `D, S, V`          | `VVVVVVVVSSDD0000` | Add sign-extended immediate value to value at source register and move it to destination register|
|`MOV`| `D, S`          | `00000000SSDD0000` | Move value at source register to destination register|
|`INC`| `D`          | `00000001DDDD0000` | Increment value at destination register|
|`DEC`| `D`          | `11111111DDDD0000` | Decrement value at destination register|

Retro-compatibility is kept for assembly code, not for machine code.

## Instruction LDV
LDV is renamed as MVV and LDV, MVI, ADI, MUI and AUI become pseudos using MVV

### Before:
|LDV| D, V | VVVVVVVVVVDD0001 | Load a value into destination register.

### After:
|LDV| D, V, O | VVVVVVVVOODD0001 | Load a value into destination register.
|MVI| D, V | VVVVVVVV00DD0001 | Set a zero-extended lower byte to destination register.
|ADI| D, V | VVVVVVVV01DD0001 | Set a zero-extended lower byte to destination register.
|MUI| D, V | VVVVVVVV10DD0001 | Set a byte left shifted by 8 bits to destination register.
|AUI| D, V | VVVVVVVV11DD0001 | Add a byte left shifted by 8 bits to destination register.

Retro-compatibility is kept for assembly code, not for machine code.

…and francisrstokes#33

- Previously LDM, renamed for STA  (STore at Address).
- LDM kept as pseudo for retro-compatibility.