Okular testing results in README

[win8linux]

Added results from testing the game PDF in Okular 21.04.0, using Normal performance mode. Should be noted that there is a white loading screen with the app icon while initially rendering a page that isn't cached:

Performance may vary based on performance mode setting:

• Low (no caching)
• Normal (preload next page and searches)
• Aggressive (preload next pages and searches)
• Greedy (preload all pages and searches)

No observed visual artifacts, unless if the loading screen counts.

[Lucas-C]

Awesome, thank you for your contribution!