update CHANGELOG and README for 0.15.0 release

[nlu90]

(If this PR fixes a github issue, please add Fixes #<xyz>.)

Fixes #

(or if this PR is one task of a github issue, please add Master Issue: #<xyz> to link to the master issue.)

Master Issue: #

Motivation

Explain here the context, and why you're making that change. What is the problem you're trying to solve.

Modifications

Describe the modifications you've done.

Verifying this change

• [ ] Make sure that the change passes the CI checks.

(Please pick either of the following options)

This change is a trivial rework / code cleanup without any test coverage.

(or)

This change is already covered by existing tests, such as (please describe tests).

(or)

This change added tests and can be verified as follows:

(example:)

• Added integration tests for end-to-end deployment with large payloads (10MB)
• Extended integration test for recovery after broker failure

Documentation

Check the box below.

Need to update docs?

• [ ] doc-required

  (If you need help on updating docs, create a doc issue)

• [ ] no-need-doc

  (Please explain why)

• [X] doc

  (If this PR contains doc changes)