[usecase-embeddingapi] Add readMe.md for embeddingAPI usecase


Impacted tests(approved): new 0, update 0, delete 0
Unit test platform: [Android]
Unit test result summary: pass 0, fail 0, block 0
There exists `usecase/embeddingapi-usecase-tests/README`. Can you combine them into `usecase/embeddingapi-usecase-tests/README.md`?

I don't think it is a good idea to keep 2 README files in one test suite.