Let's add a nice file that defines the available release channels.
Upcoming pmbootstrap patches will make use of this in 'pmbootstrap init'
for example:
[20:46:22] Choose the postmarketOS release channel.
[20:46:22] Available (2):
[20:46:22] * edge: Rolling release channel
[20:46:22] * stable: Upcoming beta release (WIP, DO NOT USE!)
[20:46:22] Channel [edge]:
Closes: #519