chore(secretmanager): adding missed header in README

[moelasmar]

This PR is to fix the readme, and add back a header that got deleted by mistake in this PR https://github.com/aws/aws-cdk/pull/26329/files#diff-c70c65923cf81d0d8bdd04d2b6160ec67aa5556be569ac08f44359df53de8bb9

Checklist

• My code adheres to the CONTRIBUTING GUIDE and DESIGN GUIDELINES

---

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.77%. Comparing base (2e3b2ac) to head (fb2a5e3).

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #32532   +/-   ##
=======================================
  Coverage   78.77%   78.77%           
=======================================
  Files         108      108           
  Lines        7163     7163           
  Branches     1320     1320           
=======================================
  Hits         5643     5643           
  Misses       1335     1335           
  Partials      185      185           

Flag	Coverage Δ
suite.unit	78.77% <ø> (ø)

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Components	Coverage Δ
packages/aws-cdk	78.77% <ø> (ø)

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AWS CodeBuild CI Report

• CodeBuild project: AutoBuildv2Project1C6BFA3F-wQm2hXv2jqQv
• Commit ID: fb2a5e3
• Result: SUCCEEDED
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