bring back the obsolete downloads

Apparently, there are still some users so bring them back as
releases in a separate repository.

docs/downloads.md

@@ -50,6 +50,8 @@ Date       | Release
 2014-06-26 | [0.12.1](https://github.com/wkhtmltopdf/wkhtmltopdf/releases/0.12.1/)
 2014-02-06 | [0.12.0](https://github.com/wkhtmltopdf/wkhtmltopdf/releases/0.12.0/)
 
+If you need versions older than `0.12.0`, you can look at the [obsolete downloads](https://github.com/wkhtmltopdf/obsolete-downloads/blob/master/README.md).
+
 ## FAQ
 
 #### Where are all the "distro-specific" binaries?