update readme

[tnguyen14]

• [x] reflect the new usage with generator-tobiko
• [x] update filepath to url example (ref https://github.com/tnguyen14/tringuyen/commit/517d7d33ca567f81a423e04e8600cd0dfe310a83)
• [x] add reference to filename and fileext (https://github.com/tnguyen14/tobiko/commit/2f23a6e8c6becf3b66486b16eb7d0f42a719f206)
• [ ] add example for templating with content
• [x] wordpress