Add serialization docs to jit/README (#17951)
Summary:
Documents the serialization format for `torch.jit.save`. Some of the info is copied from houseroad's internal doc.
[Formatted Markdown](https://github.com/driazati/pytorch/blob/serial_docs/torch/csrc/jit/README.md)
Also refactors the readme to have a heading hierarchy + table of contents
Pull Request resolved: https://github.com/pytorch/pytorch/pull/17951
Differential Revision: D14531644
Pulled By: driazati
fbshipit-source-id: cbcd9462054cc9f8a2f8cea2c98d8aba4e7d227c
