Add "Troubleshooting" link to install guide

[milesfrain]

purescript/documentation#323 (comment)

[hdgarrood]

I'd rather have the npm installer print this link if it fails. That way, if it does fail, the info you need is immediately right there; you don't have to go back to the purescript.org webpage to find it.

[milesfrain]

If you point me to the section in the installer of where to add this, I'll attempt to make that change.

[hdgarrood]

I think here would be the best place for it: https://github.com/purescript/npm-installer/blob/9761cd2778853bd661761990297fc80fa6e3755b/index.js#L315-L344

I think it should be fine to unconditionally append a couple of newlines and then this link to erroredTask.message after that property has been set initially, but if you could try to test it with each of the branches of the if statement which starts as if (err.code === 'ERR_UNSUPPORTED_PLATFORM' ... then that would be ideal.