CONTRIBUTING.md: check for whitespace before submitting

Whitespace errors are easy to create but hard to check manually. Add
two git commands that detect these whitespace errors to the commit
cleanup section of CONTRIBUTING.md.

Signed-off-by: Derrick Stolee <[email protected]>

Author: derrickstolee

CONTRIBUTING.md

@@ -316,6 +316,7 @@ Here are some other tips that we use when cleaning up our commits:
 * Make sure the commits are signed off using `git commit (-s|--signoff)`. See
   [SubmittingPatches](https://github.com/git/git/blob/v2.8.1/Documentation/SubmittingPatches#L234-L286)
   for more details about what this sign-off means.
+* Check for whitespace errors using `git diff --check [base]...HEAD` or `git log --check`.
 * Run `git rebase --whitespace=fix` to correct upstream issues with whitespace.
 * Become familiar with interactive rebase (`git rebase -i`) because you will be reordering,
   squashing, and editing commits as your patch or series of patches is reviewed.