docs: Rename readme.md to README.md

* Improve visibility and standardization across various
  platforms and tools.
* Ensure better compatibility and automatic recognition by
  GitLab and other services.
* Enhance readability and makes the README file more
  prominent in the repository.
* Add disclaimer
* Format headers and snippets using standard syntax.

Signed-off-by: Hugues Kamba-Mpiana <hugues.kambampiana@arm.com>