Update README.markdown

Author: tabatkins

README.markdown

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 ### [CSS-TAG Houdini Task Force](https://wiki.css-houdini.org/) Specifications
 
-To download a copy of all specifications:
+This is the repository containing all the CSS/TAG Houdini Task Force specifications.
 
-        hg clone https://hg.css-houdini.org/drafts
+In addition to this git repository, a Mercurial mirror is maintained at `https://hg.css-houdini.org/drafts`, if for whatever reason you prefer Mercurial.
 
-For help using or setting up Mercurial see instructions [here](http://wiki.csswg.org/tools/hg).  Specification issues are discussed on the
-public [public-houdini mailing list](http://lists.w3.org/Archives/Public/public-houdini/).
+Specification issues are raised and discussed in GitHub Issues in this repository.
 
+We also maintain the [public-houdini mailing list](http://lists.w3.org/Archives/Public/public-houdini/) for general-interest topics.