docs: add deprecation policy to README (#1428)

### What does this PR do?

Type of change: documentation

Adds a **Deprecation Policy** section to `README.md`, modeled after
[TensorRT-LLM's
policy](https://github.com/NVIDIA/TensorRT-LLM#deprecation-policy).
Since Model Optimizer is still pre-1.0 (vs. TRT-LLM being past 1.0), the
migration period is shortened from 3 months to 1 release (~1 month), and
removals may land in minor version bumps while we remain in 0.x.

### Usage

N/A — documentation-only change.

### Testing

N/A — markdownlint passes via pre-commit.

### Before your PR is "*Ready for review*"

Make sure you read and follow [Contributor
guidelines](https://github.com/NVIDIA/Model-Optimizer/blob/main/CONTRIBUTING.md)
and your commits are signed (`git commit -s -S`).

Make sure you read and follow the [Security Best
Practices](https://github.com/NVIDIA/Model-Optimizer/blob/main/SECURITY.md#security-coding-practices-for-contributors)
(e.g. avoiding hardcoded `trust_remote_code=True`, `torch.load(...,
weights_only=False)`, `pickle`, etc.).

- Is this change backward compatible?: ✅
- If you copied code from any other sources or added a new PIP
dependency, did you follow guidance in `CONTRIBUTING.md`: N/A
- Did you write any new necessary tests?: N/A
- Did you update
[Changelog](https://github.com/NVIDIA/Model-Optimizer/blob/main/CHANGELOG.rst)?:
N/A
- Did you get Claude approval on this PR?: ❌

### Additional Information

Reference: https://github.com/NVIDIA/TensorRT-LLM#deprecation-policy

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Documentation**
* Added a Deprecation Policy section to the README documenting how
deprecated features are communicated, the approximate 1-month migration
period, the scope of deprecations covered, and the timeline for feature
removal relative to semantic versioning.

[![Review Change
Stack](https://storage.googleapis.com/coderabbit_public_assets/review-stack-in-coderabbit-ui.svg)](https://app.coderabbit.ai/change-stack/NVIDIA/Model-Optimizer/pull/1428)

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Signed-off-by: Keval Morabia <28916987+kevalmorabia97@users.noreply.github.com>

Author: kevalmorabia97

README.md

@@ -137,6 +137,15 @@ more fine-grained control on installed dependencies or for alternative docker im
 | Distillation | [View Support Matrix](./examples/llm_distill/README.md#support-matrix) |
 | Speculative Decoding | [View Support Matrix](./examples/speculative_decoding/README.md#support-matrix) |
 
+## Deprecation Policy
+
+Model Optimizer follows a structured approach to managing deprecated features:
+
+- **Communication:** Deprecation notices are documented in the [Changelog](https://nvidia.github.io/Model-Optimizer/reference/0_changelog.html). Deprecated items include source code statements indicating deprecation timing, with runtime warnings issued upon use.
+- **Migration Period:** Since Model Optimizer is still pre-1.0, we provide a 1-release (~1-month) migration period after deprecation. During this window, deprecated features continue functioning while issuing warnings.
+- **Scope:** The policy addresses both complete deprecations (entire APIs removed) and partial ones (specific parameters removed while methods remain).
+- **Removal:** Following the migration period, deprecated elements are removed in alignment with semantic versioning standards, potentially including breaking changes in minor version updates while Model Optimizer remains in 0.x.
+
 ## Contributing
 
 Model Optimizer is now open source! We welcome any feedback, feature requests and PRs.