Describe markdown HTML rendering and display a warning

Fixes #688

plugins/markdown/README.md

@@ -41,6 +41,23 @@ If the tag `.nomarkdown` is set for a shaare, it won't be converted to Markdown
  
 > Note: it's a private tag (leading dot), so it won't be displayed to visitors.
 
+### HTML rendering
+
+Markdown support HTML tags. For example:
+
+    > <strong>strong</strong><strike>strike</strike>
+   
+Will render as:
+
+> <strong>strong</strong><strike>strike</strike>
+
+If you want to shaare HTML code, it is necessary to use inline code or code blocks.
+  
+**If your shaared descriptions containing HTML tags before enabling the markdown plugin, 
+enabling it might break your page.**
+
+> Note: HTML tags such as script, iframe, etc. are disabled for security reasons.
+
 ### Known issue
 
 #### Redirector

plugins/markdown/markdown.meta

@@ -1 +1,4 @@
-description="Render shaare description with Markdown syntax."
+description="Render shaare description with Markdown syntax.<br><strong>Warning</strong>:
+If your shaared descriptions containing HTML tags before enabling the markdown plugin,
+enabling it might break your page.
+See the <a href=\"https://github.com/shaarli/Shaarli/tree/master/plugins/markdown#html-rendering\">README</a>."