[GenAI] Add readme to Microsoft.ML.GenAI.Phi

[LittleLittleCloud]

We are excited to review your PR.

So we can do the best job, please check:

• There's a descriptive title that will make sense to other developers some time from now.
• There's associated issues. All PR's should have issue(s) associated - unless a trivial self-evident change such as fixing a typo. You can use the format Fixes #nnnn in your description to cause GitHub to automatically close the issue(s) when your PR is merged.
• Your change description explains what the change does, why you chose your approach, and anything else that reviewers should know.
• You have included any necessary tests in the same PR.

[luisquintanilla]

Looks great @LittleLittleCloud. Added a few suggestions.

[luisquintanilla]

For the ones that aren't checked, do we have a path to support them? If not, maybe remove them

[LittleLittleCloud]

Yes, I am working on implementing those models