Add information to README.md about using NVM

[Drewdan]

Hey Guys!

I tried out the spatie/markdown package yesterday and I got stuck on why it would not render properly. It transpires the issue was that I use NVM, and the package was looking for node in a particular place, where it was not. It took me far longer to realize that this was what was happening, and required the help of someone in the Laracasts forum too!

My first though was to update the package to accept another variable, so the user could specify the location of their node executable. But then I decided it probably made more sense just to symlink the file instead, as the likelihood is, in production, node would be installed instead of NVM, so it's less of an issue.

I figured it would be good to add this to the readme, to help anyone else that gets stuck. If this PR is accepted, I will also make a similar on one the spatie/markdown package too!

Thanks Guys :)

[riasvdv]

Thanks!