Add discontinuation notice to README

[manindhra1412]

PR summary

Adding End of Support Notice for SDK from December 15

Fixes: https://github.ibm.com/project-fortress/pm/issues/22424

PR Checklist

Please make sure that your PR fulfills the following requirements:

• The commit message follows the Angular Commit Message Guidelines.
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• New tests
• Build/CI related changes
• Documentation content changes
• Other (please describe)

What is the current behavior?

What is the new behavior?

Does this PR introduce a breaking change?

• Yes
• No

Other information