[FX] Added how to write transformations section (#51278)
Summary:
I still need to add links to vmap/grad/decomposition, but those haven't been added to the examples folder yet.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/51278
Reviewed By: zou3519
Differential Revision: D26223103
Pulled By: Chillee
fbshipit-source-id: 3ad9bf76cd3438743edecdc17c44f8d1e00e5ea1
