Add specific instructions on how to install devkit

[soundasleep]

It's not clear when downloading the devkit, what your next steps should be - just extracting it to a folder (the default) is not enough. You also have to run devkitvars.bat to add the location of the devkit to your path.

This change adds instructions on the Download page to help new users out.

[larskanis]

Thanks for notifying about this issue! In the meantime the description to the old devkit has been removed, so I'm closing this.

[alice420]

✌