headerid plugin is less relevant for Markdown

Clarifies README so it's clear that Python-Markdown library can be used
to generate link anchors without using this plugin.

headerid/README.rst

@@ -1,4 +1,13 @@
 Pelican ``headerid`` plugin
 ===========================
 
-This plugin adds an anchor to each heading so you can deeplink to headers.
+This plugin adds an anchor to each heading so you can deep-link to headers.
+It is intended for formats such as reStructuredText that do not natively
+generate these anchors.
+
+For Markdown, this plugin is less relevant since the Python-Markdown library
+includes a Table of Contents extension that will generate link anchors.
+To enable the ``toc`` extension, add a line similar to the following example
+to your Pelican settings file::
+
+    MD_EXTENSIONS = ["codehilite(css_class=highlight)", "extra", "toc"]