Document in the README.md introduction that perfanno can profile nvim itself

Hey, It's hard to find the information that this can profile nvim itself from reading quickly through the doc (what 90% of users do when trying to find whether they should install a package). We should document that in the intro. I didn't use this document I'm of clear need of because I didn't understand what it did. Also add a newline to make the intro more readable.