Missing documentation on valid modifiers for move_mask, resize_mask

[Coleman McFarland]

Resize masks and move masks were reworked in this PR:

#146

But the man page (and website?) could be a bit better.

The current man page looks like this:

       move_mask Mod1/Mod2/Mod3/Mod4/Mod5
              Determine the button mask used to move windows using the mouse.

       resize_mask Mod1/Mod2/Mod3/Mod4/Mod5
              Determine the button mask  used  to  resize  windows  using  the  mouse.   If
              move_mask and resize_mask use the same value, the move_mask takes precedence.`

But technically "Mod1", and the others listed, are invalid. Only lower-case strings (e.g. "mod1") are accepted.

[Coleman McFarland]

Fixed in #178