Rewrite README with consistent structure

Replace sparse README (no extension) with a proper README.md following
the standard plugin documentation format.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: JohnMcLear

README

@@ -0,0 +1,13 @@
+# ep_subscript_and_superscript
+
+Adds subscript and superscript text formatting buttons to the Etherpad toolbar, with HTML export support.
+
+## Install
+
+```
+pnpm run plugins i ep_subscript_and_superscript
+```
+
+## License
+
+Apache-2.0