Add notes on the use of Slack (#9442)

Author: terrytangyuan

README.md

@@ -127,5 +127,6 @@ If you use vLLM for your research, please cite our [paper](https://arxiv.org/abs
 
 * For technical questions and feature requests, please use Github issues or discussions.
 * For discussing with fellow users, please use Discord.
+* For coordinating contributions and development, please use Slack.
 * For security disclosures, please use Github's security advisory feature.
 * For collaborations and partnerships, please contact us at vllm-questions AT lists.berkeley.edu.