README link to user manual is broken

[bbolker]

This link from the README gives a 404 error.

This appears to work, sort of:

https://htmlpreview.github.io/?https://github.com/nimble-dev/nCompiler/blob/master/UserManual/nCompiler_user_manual/_book/index.html

Maybe there's a bookdown site somewhere? I poked around but couldn't find one ...