Address gatsby-plugin-feed-mdx initialization warning

[ozbe]

To avoid future breakage (and practice the discipline of not having warnings) fix the following warning that is output when gatsby build (or develop or serve) command is run:

warn gatsby-plugin-feed-mdx was initialized in gatsby-config.js without a feeds option.
This means that the plugin will use the internal RSS feed creation, which may not match your use
case.
This behavior will be removed in the next major release of gatsby-plugin-feed.

[ozbe]

See rss feed instructions for information on resolving issue.