Add CONTRIBUTING and CODE_OF_CONDUCT files (contiv#1)

We may want to centralize this information somewehere now that we have
multiple repos.

Signed-off-by: Antonin Bas <[email protected]>

Author: antoninbas

CODE_OF_CONDUCT.md

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+# Community Code of Conduct
+
+Project Antrea follows the [CNCF Code of Conduct](https://github.com/cncf/foundation/blob/master/code-of-conduct.md).

CONTRIBUTING.md

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+# Developer Guide
+
+Thank you for taking the time out to contribute to project Antrea!
+
+## Contribute
+
+### Inclusive Naming
+
+For symbol names and documentation, do not introduce new usage of harmful
+language such as 'master / slave' (or 'slave' independent of 'master') and
+'blacklist / whitelist'. For more information about what constitutes harmful
+language and for a reference word replacement list, please refer to the
+[Inclusive Naming Initiative](https://inclusivenaming.org/).
+
+We are committed to removing all harmful language from the project. If you
+detect existing usage of harmful language in code or documentation, please
+report the issue to us or open a Pull Request to address it directly. Thanks!
+
+### Sign-off Your Work
+
+As a CNCF project, Antrea must enforce the [Developer Certificate of
+Origin](https://developercertificate.org/) (DCO) on all Pull Requests. We
+require that for all commits constituting the Pull Request, the commit message
+contains the `Signed-off-by` line with an email address that matches the commit
+author. By adding this line to their commit messages, contributors *sign-off*
+that they adhere to the requirements of the DCO.
+
+Git provides the `-s` command-line option to append the required line
+automatically to the commit message:
+
+```bash
+git commit -s -m 'This is my commit message'
+```
+
+For an existing commit, you can also use this option with `--amend`:
+
+```bash
+git commit -s --amend
+```
+
+If more than one person works on something it's possible for more than one
+person to sign-off on it. For example:
+
+```bash
+Signed-off-by: Some Developer [email protected]
+Signed-off-by: Another Developer [email protected]
+```
+
+We use the [DCO Github App](https://github.com/apps/dco) to enforce that all
+commits in a Pull Request include the required `Signed-off-by` line. If this is
+not the case, the app will report a failed status for the Pull Request and it
+will be blocked from being merged.
+
+Compared to our earlier CLA, DCO tends to make the experience simpler for new
+contributors. If you are contributing as an employee, there is no need for your
+employer to sign anything; the DCO assumes you are authorized to submit
+contributions (it's your responsibility to check with your employer).