Add xmllint linter (#559)

* Add xmllint linter for xml

README.md

@@ -115,6 +115,7 @@ name. That seems to be the fairest way to arrange this table.
 | Vim | [vint](https://github.com/Kuniwak/vint) |
 | Vim help^ | [proselint](http://proselint.com/)|
 | XHTML | [proselint](http://proselint.com/)|
+| XML | [xmllint](http://xmlsoft.org/xmllint.html/)|
 | YAML | [yamllint](https://yamllint.readthedocs.io/) |
 
 * *^ No linters for text or Vim help filetypes are enabled by default.*