Update GitHub Pages installation instructions

Author: mmistakes

README.md

@@ -107,6 +107,8 @@ plugins:
   - jekyll-remote-theme
 ```
 
+Remove any other `theme:` or `remote_theme:` entry.
+
 **Note:** Your Jekyll site should be viewable immediately at <http://USERNAME.github.io>. If it's not, you can force a rebuild by **Customizing Your Site** (see below for more details).
 
 If you're hosting several Jekyll based sites under the same GitHub username you will have to use Project Pages instead of User Pages. Essentially you rename the repo to something other than **USERNAME.github.io** and create a `gh-pages` branch off of `master`. For more details on how to set things up check [GitHub's documentation](https://help.github.com/articles/user-organization-and-project-pages/).