Added note about putting in a template for the `<img>` sting.
Added note about putting in a template for the `<img>` sting.

alanwsmith/document.write-image-loader,alanwsmith/document.write-image-loader,alanwsmith/doc-write-image-loader,alanwsmith/doc-write-image-loader,alanwsmith/doc-write-image-loader