[Internal] Add DCO guidelines (#1047)

## Changes

This PR updates the contributing guidelines to include the DCO
(Developer Certificate of Origin) that external contributors must
sign-off to contribute.

## Tests

N/A

Author: renaudhartert-db

CONTRIBUTING.md

@@ -2,4 +2,24 @@ Required development tools:
 
 * `go install golang.org/x/tools/cmd/goimports@latest`
 * `go install honnef.co/go/tools/cmd/[email protected]`
-* `go install gotest.tools/gotestsum@latest`
+* `go install gotest.tools/gotestsum@latest`
+
+## Developer Certificate of Origin
+
+To contribute to this repository, you must sign off your commits to certify 
+that you have the right to contribute the code and that it complies with the 
+open source license. The rules are pretty simple, if you can certify the 
+content of [DCO](./DCO), then simply add a "Signed-off-by" line to your 
+commit message to certify your compliance. Please use your real name as 
+pseudonymous/anonymous contributions are not accepted.
+
+```
+Signed-off-by: Joe Smith <[email protected]>
+```
+
+If you set your `user.name` and `user.email` git configs, you can sign your 
+commit automatically with `git commit -s`:
+
+```
+git commit -s -m "Your commit message"
+```

DCO

@@ -0,0 +1,25 @@
+Developer's Certificate of Origin 1.1
+
+By making a contribution to this project, I certify that:
+
+(a) The contribution was created in whole or in part by me and I
+    have the right to submit it under the open source license
+    indicated in the file; or
+
+(b) The contribution is based upon previous work that, to the best
+    of my knowledge, is covered under an appropriate open source
+    license and I have the right under that license to submit that
+    work with modifications, whether created in whole or in part
+    by me, under the same open source license (unless I am
+    permitted to submit under a different license), as indicated
+    in the file; or
+
+(c) The contribution was provided directly to me by some other
+    person who certified (a), (b) or (c) and I have not modified
+    it.
+
+(d) I understand and agree that this project and the contribution
+    are public and that a record of the contribution (including all
+    personal information I submit with it, including my sign-off) is
+    maintained indefinitely and may be redistributed consistent with
+    this project or the open source license(s) involved.