fix(rss): generate slug for rss

gatsby-node.js

@@ -1,6 +1,7 @@
 const _ = require("lodash")
 const Promise = require('bluebird')
 const path = require('path')
+const { createFilePath } = require('gatsby-source-filesystem')
 
 exports.createPages = ({ graphql, boundActionCreators }) => {
   const { createPage } = boundActionCreators
@@ -42,3 +43,16 @@ exports.createPages = ({ graphql, boundActionCreators }) => {
     )
   })
 }
+
+exports.onCreateNode = ({ node, boundActionCreators, getNode }) => {
+  const { createNodeField } = boundActionCreators
+
+  if (node.internal.type === `MarkdownRemark`) {
+    const value = createFilePath({ node, getNode })
+    createNodeField({
+      name: `slug`,
+      node,
+      value,
+    })
+  }
+}