Tell jekyll to ignore .accessibility-cache

So that its local web server (`--incremental` mode)
doesn't trigger after every page gets accessibility-checked.

Author: ctrueden

_config.yml

@@ -1,3 +1,6 @@
+exclude:
+  - .accessibility-cache
+
 name: ImageJ Wiki
 title: ImageJ Wiki
 description: The ImageJ wiki is a community-edited knowledge base on topics relating to ImageJ, a public domain program for processing and analyzing scientific images, and its ecosystem of derivatives and variants, including ImageJ2, Fiji, and others.