added include plugin

plugin = @gerhobbelt/markdown-it-include
It works but it won't look in the same folder as the file that is including it.
So you might need to specify an absolute path.

nodes/lib/serveMarkdown.js

@@ -80,6 +80,7 @@ module.exports = function serveMarkdown(RED, node){
     .use(require('@gerhobbelt/markdown-it-attrs'))
     .use(require('@gerhobbelt/markdown-it-footnote'))
     .use(require('@gerhobbelt/markdown-it-emoji'))
+    .use(require('@gerhobbelt/markdown-it-include'))
     .use(require('markdown-it-playground'))
     .use(require('markdown-it-anchor'))
     .use(require('markdown-it-table-of-contents'))

package.json

@@ -22,6 +22,7 @@
     "@gerhobbelt/markdown-it-attrs": "^3.0.3-19",
     "@gerhobbelt/markdown-it-emoji": "^1.2.1-5",
     "@gerhobbelt/markdown-it-footnote": "^3.0.2-3",
+    "@gerhobbelt/markdown-it-include" : "^1.1.3-7",
     "express": "^4.17.1",
     "fs-extra": "^8.1.0",
     "handlebars": "^4.7.2",