docs: Add End of Life for Mbed notice

* The Mbed platform and OS will reach end of life in July 2026,
  when the Mbed website will be archived and it will no longer
  be possible to build projects in our online tools.
* The device software - Mbed OS - is open source and will remain
  publicly available, but is no longer actively maintained by Arm
* The Mbed TLS project is unaffected by this announcement and
  continues to be supported as part of the TrustedFirmware
  community project.

Signed-off-by: Hugues KAMBA MPIANA <hugues.kambampiana@arm.com>

README.md

@@ -3,6 +3,14 @@
 [![Build status master][mbed-master-svg]][mbed-master]
 [![Tools coverage status][mbed-coveralls-tools-svg]][mbed-coveralls-tools]
 
+---
+
+__📣Important update:__
+
+*_Arm Announces End of Life Timeline for Mbed. [Read the full announcement][eof-full-announcement]_*.
+
+---
+
 [mbed-os-logo]: logo.png
 [mbed-os-link]: https://www.mbed.com/en/platform/mbed-os/
 [mbed-master]: https://github.com/ARMmbed/mbed-os/actions/workflows/basic_checks.yml
@@ -48,3 +56,5 @@ To contribute to this documentation, please see the [mbed-os-5-docs repository](
 Please note that if you intend to use Mbed OS in a real product then you should consider the security implications of your application. Mbed OS provides user hooks (functions prefixed with WEAK symbol) that are intended to be overridden. We recommend that you carefully consider the threat model of your application and override the default user hooks provided by Mbed OS to fit your application's security needs.
 
 For example, Mbed OS executes [`mbed_die`](https://github.com/ARMmbed/mbed-os/blob/master/platform/source/mbed_board.c#L26) when there is an error. `mbed_die` by default halts the system. A production application should override weakly linked `mbed_die` function and provide own implementation suitable for their needs taking care of any security vulnerabilities and production considerations. 
+
+[eof-full-announcement]: https://os.mbed.com/blog/entry/Important-Update-on-Mbed/