add DCO to contributing template (opendistro-for-elasticsearch#15)

CONTRIBUTING.md

@@ -19,6 +19,43 @@ reported the issue. Please try to include as much information as you can. Detail
 * Any modifications you've made relevant to the bug
 * Anything unusual about your environment or deployment
 
+## Sign your work
+The sign-off is a simple line at the end of each commit, which certifies that you wrote it or otherwise have the right to pass it on as an open-source patch. if you can certify the below
+```
+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.
+```
+then you just add a line to every git commit message:
+```
+Signed-off-by: Bob Sanders <[email protected]>
+```
+You can sign off your work easily by adding the configuration in github
+```
+git config user.name "Bob Sanders"
+git config user.email "[email protected]"
+```
+Then, you could sign off commits automatically by adding `-s` or `-=signoff` parameter to your usual git commits commands. e.g.
+```
+git commit -s -m "my first commit"
+```
+
 
 ## Contributing via Pull Requests
 Contributions via pull requests are much appreciated. Before sending us a pull request, please ensure that: