Installation doc add npm --unsafe-perm

[zxygentoo]

On some Linux/BSD distros, npm likely to raise permission errors.

Maybe in the doc we can tell user if

npm install -g bs-platform

running into permission errors, they can try

npm install -g bs-platfrom --unsafe-perm

cheers.

[gladimdim]

I set npm global default folder to current user dir. It solves permissions errors too.