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- change references to pytorch-transformers to transformers
- link to code formatting guidelines

- add 'your contribution' section
- add code formatting link to 'additional context'

Prefer that general questions are asked on Stack Overflow

Streamlines usages of pytorch-transformers and pytorch-pretrained-bert. Add link to the README for the migration guide.

cc @julien-c @thomwolf

ping @lysandrejik

cc @julien-c @@thomwolf

cc @julien-c

T5WithLMHeadModel's doc string claims that indices of -1 are
ignored while computing the cross-entropy loss in the forward
pass; however, indices of -1 throw an error while indices of -100
are ignored. This commit updates the doc string to be consistent
with the class's behavior.