created gatsby-plugin-feed default for mdx (gatsbyjs#137)

packages/gatsby-plugin-mdx/feed.js

@@ -0,0 +1,65 @@
+/**
+ * Configuration for `gatsby-plugin-feed` equivalent to the default, but for MDX
+ * instead of remark.
+ *
+ * Original default: https://github.com/gatsbyjs/gatsby/blob/master/packages/gatsby-plugin-feed/src/internals.js#L20
+ * Usage: gatsby-config.js
+ *
+ * ```
+ * const mdxFeed = require("gatsby-mdx/feed");
+ *
+ * module.exports = {
+ *   plugins: [
+ *     {
+ *       resolve: `gatsby-plugin-feed`,
+ *       options: mdxFeed
+ *     }
+ *   ]
+ * };
+ * ```
+ *
+ */
+module.exports = {
+  /**
+   * no need to specify the other options, since they will be merged with this
+   */
+  feeds: [
+    {
+      serialize: ({ query: { site, allMdx } }) => {
+        return allMdx.edges.map(edge => {
+          return {
+            ...edge.node.frontmatter,
+            description: edge.node.excerpt,
+            url: site.siteMetadata.siteUrl + edge.node.fields.slug,
+            guid: site.siteMetadata.siteUrl + edge.node.fields.slug
+          };
+        });
+      },
+      query: `
+      {
+        allMdx(
+          limit: 1000,
+          sort: {
+            order: DESC,
+            fields: [frontmatter___date]
+          }
+        ) {
+          edges {
+            node {
+              frontmatter {
+                title
+                date
+              }
+              fields {
+                slug
+              }
+              excerpt
+            }
+          }
+        }
+      }
+    `,
+      output: `rss.xml`
+    }
+  ]
+};