Highlight differences of v5 to earlier versions on project page

After receiving bug reports in a project that Media PDF thumbnail generation isn't working anymore, it took me literally half a day to figure out that version 5 of the module works completely different than version 3 before. The module actually isn't backwards compatible.

Unfortunately I had to find this out the hard way as there seems to be no documentation at all that describes the differences and at least outlines the steps that need to be performed after upgrading to v5 to make the module work again.