[Docs] Add sequence parallel related to readme (#578)

* add sp to readme

* replace readthedocs with github readme

README.md

@@ -24,6 +24,7 @@ English | [简体中文](README_zh-CN.md)
 
 ## 🎉 News
 
+- **\[2024/04\]** Support Sequence Parallel for enabling highly efficient and scalable LLM training with extremely long sequence lengths! \[[Usage](https://github.com/InternLM/xtuner/blob/docs/docs/zh_cn/acceleration/train_extreme_long_sequence.rst)\] \[[Speed Benchmark](https://github.com/InternLM/xtuner/blob/docs/docs/zh_cn/acceleration/benchmark.rst)\]
 - **\[2024/02\]** Support [Gemma](xtuner/configs/gemma) models!
 - **\[2024/02\]** Support [Qwen1.5](xtuner/configs/qwen/qwen1_5) models!
 - **\[2024/01\]** Support [InternLM2](xtuner/configs/internlm) models! The latest VLM [LLaVA-Internlm2-7B](https://huggingface.co/xtuner/llava-internlm2-7b) / [20B](https://huggingface.co/xtuner/llava-internlm2-20b) models are released, with impressive performance!

README_zh-CN.md

@@ -23,6 +23,7 @@
 
 ## 🎉 更新
 
+- **\[2024/04\]** 支持序列并行训练策略以实现语言模型超长上下文训练！\[[文档](https://github.com/InternLM/xtuner/blob/docs/docs/zh_cn/acceleration/train_extreme_long_sequence.rst)\] \[[速度基准](https://github.com/InternLM/xtuner/blob/docs/docs/zh_cn/acceleration/benchmark.rst)\]
 - **\[2024/02\]** 支持 [Gemma](xtuner/configs/gemma) 模型！
 - **\[2024/02\]** 支持 [Qwen1.5](xtuner/configs/qwen/qwen1_5) 模型！
 - **\[2024/01\]** 支持 [InternLM2](xtuner/configs/internlm) 模型！同时，最新版的多模态大模型 [LLaVA-Internlm2-7B](https://huggingface.co/xtuner/llava-internlm2-7b) / [20B](https://huggingface.co/xtuner/llava-internlm2-20b) 发布，其表现出强大的性能！