CPPLINT.cfg: Ignore readability/todo

Ignore `readability/todo` rule. This gives an unpleasant warning to use
`TODO(username)` as part of TODO comment items. Just allow simple `TODO`
comment items.

Signed-off-by: Razvan Deaconescu <razvan.deaconescu@upb.ro>

CPPLINT.cfg

@@ -10,4 +10,5 @@ filter=-readability/casting
 filter=-runtime/int
 filter=-runtime/printf
 filter=-build/include_subdir
+filter=-readability/todo
 linelength=120