Share plugin: Add prominent warning in readme

plugins/tiddlywiki/share/readme.tid

@@ -1,6 +1,9 @@
 title: $:/plugins/tiddlywiki/share/readme
 
-This plugin provides tools to share tiddlers via URLs, comprising:
+<div class="tc-message-box">Do not install this plugin unless you understand exactly what it does</div>
+
+This experimental plugin provides tools to share tiddlers via URLs, comprising:
 
 * The ability to load a group of tiddlers from the browser location hash at startup
 * Wizard and templates to create URLs from group of tiddlers
+