Add DressMe as preferred supplier to Thalia style guide

(is this a bug? idk)

One-sentence description

Add DressMe as preferred supplier to Thalia style guide

Current behaviour / Reproducing the bug

Not applicable really

Expected behaviour

It would be nice to mention that DressMe is Thalia's preferred supplier for printed clothing at this page, under the section for committee clothing.

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information