docs: add emoji headings and a contributing guide

[setchy]

Part of the standardize-readme-and-contributing change.

What

New CONTRIBUTING.md — the ## Development section moves here verbatim (the npm test / npm run bundle commands, the test:live notes and the bundle note). Headings stay plain.

README.md — gains the approved emoji headings and a pointer to the new guide:

# 💠 Gitify for KDE Plasma
## 🚀 Install
### 🛍️ From the KDE Store
### 🧰 From source
### 🚫 Disable or uninstall
## 🤝 Contributing   <- new, links to CONTRIBUTING.md
## 📄 License

Notes

• Contributor-only content (development/testing/bundling) no longer lives in the README.
• The KDE Store badge and every install instruction are unchanged.
• The ## 🤝 Contributing pointer is new: it was not in the original README, but the README now loses its only development section, so a pointer keeps the guide discoverable. Easy to drop if you would rather it not be there.
• 💠 is the KDE brand emoji. 🛍️, 🧰 and 🚫 are one-off choices for sections outside the shared vocabulary.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud