Replace text editor from SublimeText2 to Atom in the install page.

I suggest changing text editor from SublimeText2 to Atom.

The biggest reason is their licenses.

Atom's license is MIT license. We can use freely. (And we can also avoid charging claim dialogs by SublimeText2.)

But in Linux case, I can't find install methods easily, so I leave SublimeText2 sentences as it is. (I'm happy to be replaced if you know good install methods in Linux.)