Update contributing info to include DCO

Author: Irina Nicolae

CONTRIBUTING.md

@@ -4,3 +4,9 @@ Adding new features, improving documentation, fixing bugs, or writing tutorials
 
 Bug fixes can be initiated through GitHub pull requests. When making code contributions to the Adversarial Robustness Toolbox, we ask that you follow the `PEP 8` coding standard and that you provide unit tests for the new features.
 
+This project uses [DCO](https://developercertificate.org/). Be sure to sign off your commits using the `-s` flag or adding `Signed-off-By: Name<Email>` in the commit message.
+
+### Example
+```bash
+git commit -s -m 'Informative commit message'
+```

README.md

@@ -75,6 +75,20 @@ python examples/<example_name>.py
 
 The `notebooks` folder contains Jupyter notebooks with detailed walkthroughs of some usage scenarios. 
 
+### Contributing
+
+Adding new features, improving documentation, fixing bugs, or writing tutorials are all examples of helpful contributions. Furthermore, if you are publishing a new attack or defense, we strongly encourage you to add it to the Adversarial Robustness Toolbox so that others may evaluate it fairly in their own work.
+
+Bug fixes can be initiated through GitHub pull requests. When making code contributions to the Adversarial Robustness Toolbox, we ask that you follow the `PEP 8` coding standard and that you provide unit tests for the new features.
+
+This project uses [DCO](https://developercertificate.org/). Be sure to sign off your commits using the `-s` flag or adding `Signed-off-By: Name<Email>` in the commit message.
+
+#### Example
+
+```bash
+git commit -s -m 'Informative commit message'
+```
+
 ## Citing ART
 
 If you use ART for research, please consider citing the following reference paper: