Update README.md (#43100)
Summary:
The changes are minor.
1. Add back the external links so that readers can find out more about external tools on how to accelerate PyTorch.
2. Fix typo
Pull Request resolved: https://github.com/pytorch/pytorch/pull/43100
Reviewed By: colesbury
Differential Revision: D23192251
Pulled By: mrshenli
fbshipit-source-id: dde54b7942ebff5bbe3d58ad95744c6d95fe60fe
