[Docs] Add "Community Themes" section to README

[Star-Trowa]

Hi HeliBoard maintainers!

HeliBoard's theming system is one of its standout features, and a
small community of theme creators has formed around it. Would you
consider adding a "Community Themes" section to the README to help
users discover them?

Proposed addition:

Community Themes

• heliboard-themes
  • Curated 16 themes collection (25+⭐, actively maintained by me)
• droid-tings
  • Themes for HeliBoard + Kvaesitso

Happy to open a PR with this change if you're open to it. Let me
know if there are specific criteria themes should meet to be listed
(e.g., minimum stars, license, screenshots, activity).

Thanks for the great keyboard!

PS: Here are few screenshots from heliboard-themes to get you interested :)

[Helium314]

I added a link to the "community" readme section in the app, and added the themes to the readme section.
You're welcome to do a PR to improve it!

As long as there aren't tons of theme repositories I don't have any specific inclusion criteria.

[Star-Trowa]

Thanks a lot, that works perfectly.

[nugraha-abd]

This is also what i have in mind for a while. We have a lot of great submitted themes and layouts.
My idea is to create two megathreads each for community theme and community layout, then add the link for those megathreads to the readme section.