Add clarity to Tutorial step 3

[bvaughn]

This issue was originally reported by @hori031617 via facebook/react/issues/10323

Text of original issue:

I tried tutorial.

3. Delete all files in the src/ folder of the new project.

I think that good example code here.
For example rm -f src/*
Because, I had a misunderstanding remove src directory.

Thank you.

[tricinel]

Hmmm...if we do this, perhaps it'd be worth doing for all those steps, rm -f src/* for step 3, touch src/index.css, touch src/index.js and even have npm install -g create-react-app

[bvaughn]

Would you like to work on this task, @tricinel ?

[tricinel]

@bvaughn, love to. There is however this open PR here: https://github.com/reactjs/reactjs.org/pull/154/files. I thinking the textual addition is good, but if you think it's a good idea, then I can add the shell commands as well.

Let's do it.

[tricinel]

@bvaughn, done. Let me know what you think.