CoC: explicitly take any whitespace breakage

We'll keep this document mostly in sync with the upstream; let's
help "git am" and "git show" by telling them that they may introduce
what we may consider whitespace errors.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

.gitattributes

@@ -6,6 +6,7 @@
 *.pm eol=lf diff=perl
 *.py eol=lf diff=python
 *.bat eol=crlf
+CODE_OF_CONDUCT.md -whitespace
 /Documentation/**/*.txt eol=lf
 /command-list.txt eol=lf
 /GIT-VERSION-GEN eol=lf