CONDUCT: add a CONDUCT document
This merely exists to make sure automated tools and cursory glances of the
repo see that we care about this kind of thing.
I have retained the optimistic name and refrained from using the prescriptive
term "Code".
ADD
CONDUCT.md
+# [<img src="https://avatars1.githubusercontent.com/u/3771091?v=3&s=30">](https://github.com/gonum) Working Together
+
+The Gonum project welcomes contributions from all. Contributors should foster an open and welcoming environment. When contributing or otherwise participating, please:
+
+<!-- This should always match the text in Working Together in CONTRIBUTING.md -->
+- Be friendly and welcoming
+- Be patient
+- Be thoughtful
+- Be respectful
+- Be charitable
+- Avoid destructive behavior
+
+Excerpted from the [Go conduct document](https://golang.org/conduct).

CONDUCT: add a CONDUCT document

This merely exists to make sure automated tools and cursory glances of the
repo see that we care about this kind of thing.

I have retained the optimistic name and refrained from using the prescriptive
term "Code".