Add idea in ideas.md

Author: arjun-menon

ideas.md

@@ -18,6 +18,10 @@ Flexible Static Site Generator
     * Document this behavior: with Name Registry, this will make the unique names rule non-case-sensitive, at least with `defaultNameEncode` converting all chars to lower case.
 * Top 2:
   * Add a trailing `/` slash for Markdown page dirs since: (a) if a Markdown page is turned into a dir/collection of smaller essays, this would allow that change to happen naturally/seamlessly, and (b) since many web servers including the one used by GH pages add a trailing `/` slash to directories using a 301 Redirect anyways.
+  * Caching in the GitHub action. See:
+    * https://docs.github.com/en/actions/using-workflows/caching-dependencies-to-speed-up-workflows
+    * https://github.com/actions/setup-python#caching-packages-dependencies
+    * https://jcdan3.medium.com/4-ways-to-speed-up-your-github-action-workflows-a0b08067a6c6
   * Enforce directory name uniqueness--currently multiple directories that are all without index pages can share the same name.
   * Rectify `linkName` (if necessary).
   * In addition to the `files`  field, we'll need some other fields.