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: bc8f7ff133

CONTRIBUTING.md

@@ -72,7 +72,7 @@ Here are some tips for getting your changes merged into xmonad:
   * Make sure you read the section on rebasing and squashing commits
     below.
 
-## Style guidelines
+## Style Guidelines
 
 Below are some common style guidelines that all of the core modules
 follow.  Before submitting a pull request, make sure that your code does
@@ -84,9 +84,11 @@ as well!
   * Follow the coding style of the module that you are making changes to
     (`n` spaces for indentation, where to break long type signatures, …)
 
-  * New code should not introduce any new warnings.  If you don't
-    compile your changes via stack or cabal, make sure to compile with
-    `-Wall -Werror -fno-warn-unused-do-bind -fwarn-tabs`.
+  * New code should not introduce any new warnings.  If you want to
+    check this yourself before submitting a pull request, there is the
+    `pedantic` flag, which is enforced in our CI.  You can enable it by
+    building your changes with `stack build --flag xmonad:pedantic` or
+    `cabal build --flag pedantic`.
 
   * Likewise, your code should be free of [hlint] warnings; this is also
     enforced in our GitHub CI.