Document `octarine` suite

This might also happen in the pureos wiki and we link from to there:

Octarine is an overlay suite over byzantium. The priorities are such that packes don't get pulled in automatically but need to be fetched explicity:

One time setup

cat <<EOF >/etc/apt/sources.list.d/octarine.list
deb https://repo.pureos.net/pureos octarine main
EOF

To install packages:

apt install -t octarine <pkg>

N.B. to see where pakcages would be fetched from use:

$ apt policy phosh
phosh:
  Installed: 0.15.0+next20220129.1058.1+librem5ci74614.2a75980
  Candidate: 0.15.0+next20220129.1058.1+librem5ci74614.2a75980
  Version table:
 *** 0.15.0+next20220129.1058.1+librem5ci74614.2a75980 100
        100 /var/lib/dpkg/status
     0.15.0-1pureos1 100
        100 https://repo.pureos.net/pureos octarine/main arm64 Packages
     0.14.1-1pureos1 500
        500 https://repo.pureos.net/pureos byzantium/main arm64 Packages
         99 http://repo.pureos.net/pureos landing/main arm64 Packages