Add a CODE_OF_CONDUCT.md

[AliSoftware]

We need to make contributors feel welcome to contribute to SwiftGen. Everyone should feel safe to participate to this project.

We haven't had any problems in this community so far, but it's always better to have a CoC to let everybody know that we're proactive on making the community welcoming anyway.

---

Aim is to be 100% on https://developer.github.com/changes/2017-02-09-community-health/ for all our repos

for repo in Eve SwiftGen SwiftGenKit StencilSwiftKit templates
do
  echo $repo
  curl -H "Accept: application/vnd.github.black-panther-preview+json" -H "Authorization: token $GH_API_TOKEN" "https://api.github.com/repos/SwiftGen/$repo/community/profile"
done