Add other standard doc stuff

[paddyroddy]

Related to #240. Things like

• contributing guidelines
• code of conduct
• etc.

I have some of these here https://github.com/paddyroddy/python-template

[samcunliffe]

+1 for CONTRIBUTING.md

We can pull out the developer stuff from README and put it after a nice welcoming

"hi, please chat in discussions and don't be shy about opening issues"

(Exact phrasing subject to refinement)

[samcunliffe]

[paddyroddy]

• code of conduct Create CODE_OF_CONDUCT.md #247
• contributing Create CONTRIBUTING.md #252
• issue templates Add issue templates to this project & the template #262

I'm not really a fan of PR templates. Anyone have strong views?

[samcunliffe]

Also not a fan of PR templates, so with two 👎 , I claim this is done.