Merge 2018.3 -> master

Update the project to Unity 2018.2.6f1 .

Merge branch 'new-prefab-workflow' into 2018.3 , compatible with 2018…

….3.0a10

Updated README.md with branch information for 2018.2 and 2018.3.

Update README with the version of the currently compatible Unity edit…

…or - 2018.1.

Updated README with supported version number.