docs(strlen): adding the strlen documentation

- This comment is here to explain each etape. The main goal of this is
for all people never made assembly can understand

src/ft_strlen.s

@@ -4,16 +4,16 @@ section .text
 	global ft_strlen
 
 ft_strlen:
-	xor rax, rax
-	cmp rdi, 0
-	je .exit
-	jmp .loop
+	xor rax, rax				; rax = 0
+	cmp rdi, 0					; check rdi is NULL
+	je .exit					; if rdi NULL exit
+	jmp .loop					; go to loop
 
 .loop:
-	cmp BYTE [rdi + rax], 0
-	je .exit
-	inc rax
-	jmp .loop
+	cmp BYTE [rdi + rax], 0		; check if the read byte is the end ('\0')
+	je .exit					; if it's true return to the exit
+	inc rax						; counter + 1
+	jmp .loop					; loop again
 
 .exit:
-	ret
+	ret							; return rax