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/README.md

@@ -50,4 +50,4 @@ To add a new theme, follow the following steps:
 
 Monaco themes are defined in Javascript and are stored in the `app/assets/javascripts/ide/lib/themes/` directory.
 To modify any syntax highlighting colors or to synchronize the theme colors with syntax highlighting colors, you
-can modify the files in that directory directly.
\ No newline at end of file
+can modify the files in that directory directly.

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;