apt README outdated and helpful updates

[yakatz]

The README on apt.puppet.com has a broken link to https://puppet.com/docs/puppet/latest/puppet_platform.html

The README in this repository which looks like it would be the source for that README is significantly different, but also outdated in its mentions of Jira for issue reporting.

On the subject of changes to the README, it might also be nice to show users an automatic expansion of the codename for the deb file, since people often don't remember what it is:

## Installation
To add the repo for your distribution, install the release package with the
codename for your distribution. For example, on xenial:
wget http://apt.puppetlabs.com/puppet-release-xenial.deb;
sudo dpkg -i puppet-release-xenial.deb
sudo apt-get update

On most Debian-based systems, you can get the codename automatically:
wget https://apt.puppet.com/puppet-release-$(lsb_release -cs).deb
sudo dpkg -i puppet-release-$(lsb_release -cs).deb
sudo apt update

[e-gris]

Acknowledged.

I've tweaked the local templates a bit, but also have a ticket in for the organizational settings from which this repository derives.

I'll be writing an internal ticket for the README updates.