docs: Update apt instructions (#107)

* README: Save APT repo to redbubble.list.

yak isn't the only thing we might distribute.

* README: apt-key is deprecated.

`apt-key` is deprecated; the suggestion is to manage keyring files in the /etc/apt/trusted.gpg.d
folder instead, see apt-key(8).

Let's update the README to reflect that, since folks were getting a warning when following the
previous instructions.

README.md

@@ -32,8 +32,9 @@ running:
 ```sh
 sudo apt install curl gnupg2
 # This is the Redbubble GPG key, to verify releases:
-curl https://raw.githubusercontent.com/redbubble/yak/master/static/delivery-engineers.pub.asc | sudo apt-key add -
-echo "deb http://apt.redbubble.com/ stable main" | sudo tee /etc/apt/sources.list.d/yak.list
+curl -Lq https://raw.githubusercontent.com/redbubble/yak/master/static/delivery-engineers.pub.asc | sudo gpg --no-default-keyring --import --keyring gnupg-ring:/etc/apt/trusted.gpg.d/redbubble.gpg
+sudo chmod a+r /etc/apt/trusted.gpg.d/redbubble.gpg
+echo "deb http://apt.redbubble.com/ stable main" | sudo tee /etc/apt/sources.list.d/redbubble.list
 sudo apt update
 sudo apt install yak
 ```