Add projector resolution (#1337)

Co-authored-by: Nikoś <[email protected]>

Author: cmaureir

src/content/pages/guidelines.mdx

@@ -34,6 +34,8 @@ are:
 * Try not to cram too much on one slide. There are no hard and fast rules about how many slides to
   have in a talk, as long as the slides naturally flow with your spoken narrative and don’t overwhelm
   your audience with information.
+* The resolution of the projectors is 1080p (1920 × 1080 pixels), so be mindful in case your
+  computer has a different one, like a 4k screen, so you can test that before.
 
 This [wonderful blog
 post](https://distinct-browser-08c.notion.site/purplecon-tips-for-writing-a-good-conference-talk-cab51d9bfd474e26ae4b021a7a12f38b)