added deployment distro units note

the deploy workflow's distro list needs to be updated periodiclly.

particularly, the ubuntu versions list must be kept up-to-date
with their current latest short term and long term versions.
otherwise it will break deploy workflow, because canonical moves
repos into archives fairly often. if our deploy workflow tries to
build for a ubuntu version that has been archived, it will fail.

for example, as of this commit, ubuntu19.04 repo has already been
archived, so I had to use 19.10 in the build list instead.

.github/workflows/deploy.yml

@@ -12,6 +12,10 @@ jobs:
     strategy:
       matrix:
         container: [ "ubuntu:18.04", "ubuntu:19.10", "ubuntu:20.04", "debian:9", "debian:10", "fedora:29", "fedora:30", "fedora:31", "fedora:32", "archlinux:latest" ]
+        # this list should be updated from time to time by consulting these pages:
+        # https://releases.ubuntu.com/
+        # https://wiki.debian.org/DebianReleases#Production_Releases
+        # https://fedoraproject.org/wiki/Releases
     runs-on: "ubuntu-latest"
     container:
       image: ${{ matrix.container }}