Change master in links to HEAD

Author: wooorm

readme.md

@@ -343,11 +343,11 @@ abide by its terms.
 
 [author]: https://wooorm.com
 
-[contributing]: https://github.com/syntax-tree/.github/blob/master/contributing.md
+[contributing]: https://github.com/syntax-tree/.github/blob/HEAD/contributing.md
 
-[support]: https://github.com/syntax-tree/.github/blob/master/support.md
+[support]: https://github.com/syntax-tree/.github/blob/HEAD/support.md
 
-[coc]: https://github.com/syntax-tree/.github/blob/master/code-of-conduct.md
+[coc]: https://github.com/syntax-tree/.github/blob/HEAD/code-of-conduct.md
 
 [tree]: https://github.com/syntax-tree/unist#tree
 
@@ -371,7 +371,7 @@ abide by its terms.
 
 [value]: https://github.com/syntax-tree/hast#propertyvalue
 
-[github]: https://github.com/jch/html-pipeline/blob/master/lib/html/pipeline/sanitization_filter.rb
+[github]: https://github.com/jch/html-pipeline/blob/HEAD/lib/html/pipeline/sanitization_filter.rb
 
 [xss]: https://en.wikipedia.org/wiki/Cross-site_scripting