Revamp (trim) the Mercurial documentation

Part of w3c#1844

Author: foolip

README.markdown

@@ -2,19 +2,18 @@
 
 This is the official CSSWG repository for editor's drafts of CSS specifications.
 
-This repository is a two-way mirror of a Mercurial repository:
+See the [contributor guidelines](CONTRIBUTING.md) for how to contribute.
 
-        hg clone https://hg.csswg.org/drafts
-
-For help using or setting up Mercurial 
-see [instructions](https://wiki.csswg.org/tools/hg).
-Specification issues are discussed as issues in this repo. 
+Specification issues are discussed as issues in this repository. 
 General CSSWG discussion can be found on the public 
 [www-style mailing list](https://lists.w3.org/Archives/Public/www-style/).
 
 To read the specifications in this repository, see them at the
 [index of all specifications](https://drafts.csswg.org/).
 
+This repository is a two-way mirror of https://hg.csswg.org/drafts, and
+contributing using Mercurial is also possible.
+
 Also see the related repositories:
 * [fxtf-drafts github repository](https://github.com/w3c/fxtf-drafts/) - [fxtf mercurial repository](https://hg.fxtf.org/drafts/) - [FX Task Force generated specification index](https://drafts.fxtf.org/)
 * [css-houdini-drafts github repository](https://github.com/w3c/css-houdini-drafts/) - [css-houdini mercurial repository](https://hg.css-houdini.org/drafts/) - [CSS-TAG Houdini Task Force generated specification index](https://drafts.css-houdini.org/)