Add documentation to add a new model to the repo

[mouxinyue1]

Hello, I would like to ask myself how to add a new model

[alexriedel1]

You basically have to factor your model code like one of the existing models https://github.com/openvinotoolkit/anomalib/tree/development/anomalib/models

then you have to add tests and documentation and create a pull request :)

[samet-akcay]

Addressed with #576.