add community guidelines documents

[benmarwick]

As part of the JOSS review process, I have noticed that the project is lacking Community guidelines. We do not see "clear guidelines for third parties wishing to 1) Contribute to the software 2) Report issues or problems with the software 3) Seek support"

I recommend to add

• [x] a CONTRIBUTING.md document (e.g. https://github.com/tidyverse/tidyverse/blob/master/.github/CONTRIBUTING.md) and
• [x] a CODE_OF_CONDUCT.md document (e.g. https://github.com/tidyverse/tidyverse/blob/master/.github/CODE_OF_CONDUCT.md) and
• [x] Issue templates (e.g. https://github.com/tidyverse/tidyverse/tree/master/.github/ISSUE_TEMPLATE)

[zoometh]

@benmarwick I've added the missing guidelines documents:

• CONTRIBUTING.md
• CODE_OF_CONDUCT.md
• Issue templates