Change master in links to HEAD

Author: wooorm

readme.md

@@ -146,8 +146,8 @@ abide by its terms.
 
 [is]: https://github.com/syntax-tree/unist-util-is
 
-[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