Recommend installing nvm in ~/.nvm

The README oddly recommends to install nvm inside of `~/nvm` which goes against conventions. It makes more sense to install tools like nvm in hidden home directories.

Oddly enough the existing README also claims that the shell script installation method puts nvm inside of `~/.nvm`. In the case, why do the manual installation instructions differ?

It's also best to use the `source` command explicitly instead of its `.` alias so that more people (even if they don't know bash scripting well) understand what the command does.