GitLab

- mostly stylistic streamlining
- removed 'additional context' sections. They seem to be rarely used and might cause confusion. If more details are needed, users can add them to the 'details' section

Motivate users to @-tag authors of models to increase visibility and expand the community

- 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.