Add note about using single quotes for --purs-args

[milesfrain]

It's confusing when porting commands from other tools which don't require quotes around purs args. For example:
This works:

psc-package build -- --codegen js,corefn

This does not:

spago build --purs-args --codegen js,corefn

This is required instead:

spago build --purs-args '--codegen js,corefn'

[f-f]

Thank you for this! 🙂
Why not double quotes though? Single quotes do not work on Windows..

[milesfrain]

Was basing it off of this example in the readme:

spago build --purs-args '-o myOutput/'

Ugg... so annoying you can't link to lines in a .md file.

I'll change this PR to just say "quotes"

[f-f]

Oh, thanks for noting, maybe we should change that example too 😄