Tweaked the intro page

Here are some ideas that I think work better:
- 15 repetitions of "React". Removed a few.
- The "two main ideas" weren't clearly separated. I put them under different headers and simplified the wording.
- The "Give it Five Minutes section didn't sound as reassuring. Made it sound more certain.
changing back to 'give it five minutes', tweak that sentence
grammar error
Merge pull request #199 from chenglou/patch-1

Tweaked the intro page