Remove README from static/img and replace it with a blank .gitignore

As per the [Git FAQ](https://git.wiki.kernel.org/index.php/GitFaq#Can_I_add_empty_directories.3F) a blank .gitignore is the preferred way to add an empty directory which should exist on checkout.

> If you really need a directory to exist in checkouts you should create a file in it. .gitignore works well for this purpose