CONTRIBUTING.md: Refer to pedantic
CONTRIBUTING.md: Refer to pedantic

Instead of manually compiling with `-Wall -Werror` and the like, we have
a `pedantic` flag in xmonad's cabal file with which to run the CI.
Since contributors are very unlikely to not use either stack or cabal to
build xmonad, we should refer them to this flag instead of listing all
of the options they would need to compile xmonad with in order to check
their work.

Fixes: bc8f7ff133e6a5415fd831c3ab232199ab462800

xmonad/xmonad