[jgrapht-users] code of conduct

[John S. <js...@gm...>]

In line with contemporary best practices for inclusive and respectful open
source collaboration, we've added a code of conduct for the JGraphT
community:

https://github.com/jgrapht/jgrapht/blob/master/CODE_OF_CONDUCT.md

The language is derived from the Contributor Covenant, which is a common
template for growing a welcoming community.  If you'd like to suggest any
specific improvements to the language, please feel free to open an issue in
github on this topic.

As you interact with others in our community, we appreciate your efforts in
remaining mindful of these guidelines.