[Impeller] Add section to the readme on how custom embedders can try Impeller.

@loic-sharma suggested after our conversation yesterday. I didn't realize there was [no mention of custom embedders at all here](https://github.com/flutter/flutter/blob/824868f5d1e6c091a5df23d3628407e75149e1da/engine/src/flutter/impeller/README.md#try-impeller-in-flutter).