Improve README.md to ease the onboarding process for newcomers.

[costacoz]

I've stumbled upon a hard onboarding and spent several hours, which I believe might have been saved if the documentation had the lines I've added. This would let less experienced users like myself to start using this wonderful theme with less headache. As I already saw discussion in Issues about the same, that "theme doesn't work out of the box". So the clarification in pull-request would remove such issues and let many beginners to onboard faster and easier.