Updated readme.md with some useful info and a caveats section

[Dogleader6]

Fixed #37 and added information that involved #24 as well as made some wording more clear while adding some important info.

I also made it clear that you must use the power port and in certain situations you can get an adapter to use the power supply while being connected to your pc in situations where the pc cannot power the pi (like overclocking) or if you need the pi to stay powered while your main pc is off.

[Dogleader6]

I don't believe this needs the check since it's just a readme update and not updating actual code, but please tell me if I'm wrong.