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

- Add embeddingapi-usecase-tests/src/org/xwalk/embedded/api/sample/READ.md
- Rename embeddingapi-usecase-tests/README to embeddingapi-usecase-tests/README.md
Merge pull request #972 from zhuyongyong/yongzhu/usecase-embeddingapi/addReadMe

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