Yuxiong doc edits (#38)
* Added features.md and Getting Started guide.
* trimming newlines
* Adding hostfile discussion
* include/exclude emphasis
* Edits overview
* minor edits on the overview
* minor edits
* Fixing broken GPT2 links
* Clarify our relationship with Megatron-LM
* Update README.md
* Update README.md
* minor edits
* fix formatting
Co-authored-by: Shaden Smith <ShadenTSmith@gmail.com>
Co-authored-by: yuxionghe <yuxhe@microsoft.com>
