[README] Add "big" note about why there are both FCQN in inject & imports

[stloyd]

As per comment: #1564 (comment)

The best ways to solve this is using coding standard to handle your imports.
Using FQN by default is to allow them import or keep FQN - some people prefer first, some second. And it's harder to FQNize than import in coding standard rules.

tl;dr; AST sucks in coding standard, and always will. It doesn't make sense to rewrite EasyCodingStandard, PHP CS Fixer or CodeSnffier. Just use it.

See this comment for deeper answer: #1456 (comment)

I recommend using option --with-style: https://github.com/rectorphp/rector#how-to-apply-coding-standards

This IMHO should be hardly noted in README file, so there is clear point why it's this way & why it will not be fixed.

[TomasVotruba]

I'm not the best person to do this, since I suffer author blindness.

Could you send PR so it's understanable for you as a tool user?