Add contributing guidelines

[alisonrclarke]

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[FlicAnderson]

Currently on the wiki we point people to check out project issue tickets, but I think we can do better :)

@alisonrclarke's idea about pointing people with particular skill groups to particular repo issues pages is a good one, and quite welcoming!

I think we could also add some info on things like branching (ie using issue-numbers in branch names to help keep track of these), what our ideas/aims are on whether code should be reviewed by 1+ person before merging, etc.