Issue Repo DOI

[hsteptoe]

I think this repo/project is now seeing sufficient development that we should issue it with a DOI. This would: (a) provide it with a permanent and persistent point of reference, (b) make it clear how it should be cited or referenced and (c) allow us to acknowledge all the contributors past and present who have got us to where we are today.

We already have an agreed licence for the whole repo, so this shouldn't require any further approvals. This is simply an labelling procedure.

I would suggest Zenodo as the DOI mint, as it has nice integration with github and keeps track of difference releases etc. but other mints may also be available.

Thoughts? @nhsavage @zmaalick @gredmond-mo