Add warning to ResourceMapper.

Author: RubenVerborgh

lib/resource-mapper.js

@@ -5,9 +5,18 @@ const { types, extensions } = require('mime-types')
 const readdir = promisify(fs.readdir)
 const HTTPError = require('./http-error')
 
-// A ResourceMapper maintains the mapping between HTTP URLs and server filenames,
-// following the principles of the “sweet spot” discussed in
-// https://www.w3.org/DesignIssues/HTTPFilenameMapping.html
+/*
+ * A ResourceMapper maintains the mapping between HTTP URLs and server filenames,
+ * following the principles of the “sweet spot” discussed in
+ * https://www.w3.org/DesignIssues/HTTPFilenameMapping.html
+ *
+ * This class implements this mapping in a single place
+ * such that all components use the exact same logic.
+ *
+ * There are few public methods, and we STRONGLY suggest not to create more.
+ * Exposing too much of the internals would likely give other components
+ * too much knowledge about the mapping, voiding the purpose of this class.
+ */
 class ResourceMapper {
   constructor ({
     rootUrl,