add MMYOLO desc in README

[isLinXu]

Thanks for your contribution and we appreciate it a lot. The following instructions would make your pull request more healthy and more easily got feedback.
If you do not understand some items, don't worry, just make the pull request and seek help from maintainers.

Motivation

add MMYOLO desc in README.md

Modification

add MMYOLO desc in README.md

BC-breaking (Optional)

None

Use cases (Optional)

None

Checklist

1. Pre-commit or other linting tools should be used to fix the potential lint issues.
2. The modification should be covered by complete unit tests. If not, please add more unit tests to ensure the correctness.
3. If the modification has potential influence on downstream projects, this PR should be tested with downstream projects, like MMDet or MMCls.
4. The documentation should be modified accordingly, like docstring or example tutorials.

[CLAassistant]

All committers have signed the CLA.

[ly015]

@isLinXu Thanks for your help!