chore(CONTRIBUTING.md): update contributing

Author: Marcos Cáceres

CONTRIBUTING.md

@@ -1,11 +1,10 @@
-# Web Platform Working Group
-
+# Contributing
 Contributions to this repository are intended to become part of Recommendation-track documents governed by the
-[W3C Patent Policy](http://www.w3.org/Consortium/Patent-Policy-20040205/) and
+[W3C Patent Policy](http://www.w3.org/Consortium/Patent-Policy/) and
 [Software and Document License](http://www.w3.org/Consortium/Legal/copyright-software). To make substantive contributions to specifications, you must either participate
 in the relevant W3C Working Group or make a non-member patent licensing commitment.
 
-If you are not the sole contributor to a contribution (pull request), please identify all 
+If you are not the sole contributor to a contribution (pull request), please identify all
 contributors in the pull request comment.
 
 To add a contributor (other than yourself, that's automatic), mark them one per line as follows:
@@ -20,14 +19,36 @@ If you added a contributor by mistake, you can remove them in a comment with:
 -@github_username
 ```
 
-If you are making a pull request on behalf of someone else but you had no part in designing the 
+If you are making a pull request on behalf of someone else but you had no part in designing the
 feature, you can remove yourself with the above syntax.
 
-# Tests
-
+## Tests
 For normative changes, a corresponding
 [web-platform-tests](https://github.com/web-platform-tests/wpt) PR is highly appreciated. Typically,
 both PRs will be merged at the same time. Note that a test change that contradicts the spec should
 not be merged before the corresponding spec change. If testing is not practical, please explain why
 and if appropriate [file a web-platform-tests issue](https://github.com/web-platform-tests/wpt/issues/new)
 to follow up later. Add the `type:untestable` or `type:missing-coverage` label as appropriate.
+
+# Style guide to contributors
+
+- the spec uses [ReSpec](https://www.w3.org/respec/)
+- the spec is tidied using [HTML5 Tidy](https://github.com/w3c/tidy-html5). For
+instructions on running HTML5 tidy, see below.
+- put comments in front of sections, for better readability with
+  syntax coloring editors.
+
+# Running HTML5 Tidy
+
+Please make sure you have HTML5 tidy installed, instead of
+the the one that ships with *nix systems. You can comfirm this by running:
+
+```bash
+tidy --version  #HTML Tidy for HTML5 (experimental) for ...
+```
+Once you have confirmed (make sure you have committed your changes before
+running tidy, as the changes are destructive ... in a good way:)):
+
+```bash
+tidy -config tidyconfig.txt -o index.html index.html
+```