update readme about a bug in GH-pages

README.md

@@ -1 +1,4 @@
-TODO write instructions
+TODO write instructions
+
+
+known bug (problem on GitHub Pages part) - if you have a project page and want a custom 404 page, you must use a custom domain.  See https://help.github.com/articles/custom-404-pages/.  This means that if you have a regular User Page you can use the 404 page from this theme, but if it's a website for a specific repository, the 404 page will not be used.