Use wget in instructions instead of curl

[beruic]

Curl is no longer installed by default on Ubuntu, and AFAIK neither curl nor wget is installed by default on Debian. It therefore makes sense to make most peoples experience with installing easier, by writing

wget -O- -q https://dl.yarnpkg.com/debian/pubkey.gpg | sudo apt-key add -

in instrucitons instead of

curl -sS https://dl.yarnpkg.com/debian/pubkey.gpg | sudo apt-key add -

At least for the Ubuntu/Debian instructions.

[Daniel15]

Indeed, looks like wget is installed on a lot more Debian systems compared to curl, at least according to the (opt-in) popularity contest: https://qa.debian.org/popcon-graph.php?packages=wget+curl&show_installed=on&want_legend=on&want_ticks=on&from_date=&to_date=&hlght_date=&date_fmt=%25Y-%25m&beenhere=1

Would you like to submit a pull request updating the docs?