Docs: Added licensing to contributing guide

Also renamed COPYING.TXT to LICENSE

Signed-off-by: Max Logaev <maxlogaev@proton.me>

CONTRIBUTING.md

@@ -66,6 +66,17 @@ to the end commit message body on a new line.
 
 Use **rebase** to keep your branch up to date.
 
+## Licensing
+
+For new source code files and for existing ones without a license, you need to add the following header to the beginning of the file:
+```asm
+; SPDX-License-Identifier: GPL-2.0-only
+; Program - Brief description.
+; Copyright (C) 2011-2025 KolibriOS team
+```
+
+Be careful when setting copyright and date interval. Review the file's history to verify its origin.
+
 ## Conclusion
 
 We hope this small instructions will help you to get familiar with KolibriOS contribution rules and inspire you to participate in the life of our project.

README.md

@@ -1,6 +1,6 @@
 # KolibriOS
 
-[![License](https://img.shields.io/badge/License-GPL%202.0-green)](./COPYING.TXT)
+[![License](https://img.shields.io/badge/License-GPL%202.0-green)](./LICENSE)
 [![Build system](https://git.kolibrios.org/KolibriOS/kolibrios/actions/workflows/build.yaml/badge.svg)](https://git.kolibrios.org/KolibriOS/kolibrios/actions)
 
 KolibriOS is a hobby operating system for x86-compatible computers, which is currently being developed by a small but passionate team of enthusiasts.