Cannot decode simultaneously pressed keys -> README.

As reported by freshfish47, I overlooked one complication when
reusing column/row lines for scanning. When putting out one
output line LOW to act as row-scan-line, a pressed key will of
course put the corresponding column also to low, thereby acting
as another scan-line. The quoted example is the simultaneous
closing of switch d and c. P1 is driven low.

"d" causes current to flow from line P0 to P1. "c" normally
would not be scanned right now, but with P0 been pulled low,
"c" will cause current to flow from line P3 to P0. Hence the
button normally between P1 and P3, which is "k" is erroneously
been marked as pressed.

             +====================================+
             # +-------------------------+        #
             # | +==============+        |        #
             # | # +===+        #        |        #
             # | # #   #        #a       |b       #c
     (P0)<<==#=|=#=+===#========:========:========X
             # | #     #d       #        |e       #f
  LOW(P1)>>==#=|=+=====X========#========:========:
             # |       #g       #h       |        #i
     (P2)<<--#-+-------:--------:--------|--------
             #         #j       #k       |l
     (P3)<<==+=========:========?========:

      >> Driven Low   X closed switch & diode
      << Sense        : open switch
                      ? open, but erroneously detected as closed