Add README for benchmarking SGLang using GenAI-Perf.

[ganeshku1]

Motivation

Adding GenAI-Perf a tool agnostic to hardware provides a level playing field to benchmark all GenAI end points.

Modifications

Added a Readme file to showcase how to benchmark SGLang using GenAi-Perf.

Checklist

• [x] Format your code according to the Code Formatting with Pre-Commit.
• [ ] Add unit tests as outlined in the Running Unit Tests.
• [x] Update documentation / docstrings / example tutorials as needed, according to Writing Documentation.
• [ ] Provide throughput / latency benchmark results and accuracy evaluation results as needed, according to Benchmark and Profiling and Accuracy Results.
• [ ] For reviewers: If you haven't made any contributions to this PR and are only assisting with merging the main branch, please remove yourself as a co-author when merging the PR.
• [ ] Please feel free to join our Slack channel at https://slack.sglang.ai to discuss your PR.