Document how to attach files to issues

[anjackson]

As raised in #162, it's not really clear how to attach files to this issue tracker. GitHub only supports image attachments directly, and we can use Gists for text attachments. This should be documented on the appropriate wiki page.

Binary files are a pain, but perhaps we can partially deal with this by trying to help people turn any problematic files we come across into solid test cases that can be added to the codebase and used for regression testing in future.