Complete xSDK Policies

For maximum compatibility and community-niceness, let's check what's still missing to comply fully with the [xSDK policy](https://github.com/xsdk-project/xsdk-community-policies)

- [ ] (M7) potentially consider re-licensing to BSD-like/MIT/ISC license? (weak copyleft ok so far, but might change)
- [x] (M8) Provide a runtime API to return the current version number of the software. #665
- [ ] (M11) make some leftover `std::cerr` messages controllable, also (R3) about error/exception control
- [ ] (R7) `SUPPORT.md` file in top level

Also and independent of xSDK, add
- [x] Code of Conduct: #619

- [ ] [xSDK package policies](https://github.com/xsdk-project/xsdk-community-policies/tree/master/package_policies)
- [ ] [xSDK installation policies](https://github.com/xsdk-project/xsdk-community-policies/tree/master/installation_policies)