Enhance README with welcome and contribution info

Added a welcoming message and contribution guidelines.

Author: oblomov-dev

README.md

@@ -1,3 +1,11 @@
 ## abap2UI5-docs
 
-The files of the abap2UI5 [documentation](https://abap2ui5.github.io/docs/). Feel free to contribute if you find any typos or have additional content to add.
+Welcome! 👋
+
+This repository contains the source files for the [abap2UI5 documentation](https://abap2ui5.github.io/docs/).
+
+## Contributing
+
+We appreciate your help! If you spot a typo, want to improve existing content, or have ideas for new topics, feel free to open a pull request or issue.
+
+Every contribution makes the documentation better for the community!