docs : how to add a model (#6565)

* docs: how to add a model

* docs: model: typo and docs

* docs: model: add prevision on RoPE

* docs: model: rephrasing README.md

* docs: model: rephrasing README.md

* docs: model: README.md fix trailing spaces

* docs : some fixes

* Update README.md

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Co-authored-by: Georgi Gerganov <ggerganov@gmail.com>