Update toc feature README

Author: zw963

README.md

@@ -57,7 +57,7 @@ Markd.to_html(markdown, options)
 | prettyprint | `Bool` | false         | if **true**, code tags generated by code blocks will have a `prettyprint` class added to them, to be used by [Google code-prettify](https://github.com/google/code-prettify).   |
 | gfm         | `Bool` | false         | **Partial support**                                                                                                                                                         |
 | autolink    | `Bool` | false         | if **true**, more autolinks are detected, like bare email addresses or http links. Requires `gfm` be set to `true`                                  |
-| toc         | `Bool` | false         | if **true**, show a svg link to the beginning of the head tag(as github markdown does).                                                                                                                                                       |
+| toc         | `Bool` | false         | **Partial support**, if **true**, show only a anchor text to the beginning of the head tag(as github markdown does) for now.                                                                                                                                                       |
 | base_url    | `URI?` | nil           | if not **nil**, relative URLs of links are resolved against this `URI`. It act's like HTML's `<base href="base_url">` in the context of a Markdown document.                    |
 
 ## Advanced