What is a github Milestone?

[Cleop]

From: dwyl/process-handbook#37

What are they? How are they used? Why do we use them?

[Cleop]

@iteles - do you think we should put this info here or on github-reference? This issue already exists there: dwyl/git-guide#13

----- follow up

We agreed that what a milestone is in a technical sense of creating one etc. should be written in github-reference whereas the dwyl concept of what context we use milestones in should be written in here. This is currently being included in #22