Add comments to README file

Also fixes ending newline

app/assets/stylesheets/page_bundles/_ide_theme_overrides.scss

+// -------
+// Please see `app/assets/stylesheets/page_bundles/ide_themes/README.md` for a guide on contributing new themes
+// -------
 .ide.theme-dark {
   a:not(.btn) {
     color: var(--ide-link-color);

app/assets/stylesheets/page_bundles/ide_themes/_dark.scss

+// -------
+// Please see `app/assets/stylesheets/page_bundles/ide_themes/README.md` for a guide on contributing new themes
+// -------
 .ide.theme-dark {
   --ide-border-color: #1d1f21;
   --ide-highlight-accent: #fff;