contributing: add Developer Certificate of Origin for Apache 2.0

.github/CONTRIBUTING.md

 # Contributing to `eris-db`:
 Forked from Docker's [contributing guidelines](https://github.com/docker/docker/blob/master/CONTRIBUTING.md)
 
+## Developer Certificate of Origin
+
+Please always include "signed-off-by" in your commit message and note this constitutes a developer certification you are contributing a patch under Apache 2.0.  Please find a verbatim copy of the Developer Certificate of Origin in this repository [here](.github/DEVELOPER_CERTIFICATE_OF_ORIGIN.md) or on [developercertificate.org](https://developercertificate.org/).
+
 ## Bug Reporting
 
 A great way to contribute to the project is to send a detailed report when you encounter an issue. We always appreciate a well-written, thorough bug report, and will thank you for it!

.github/DEVELOPER_CERTIFICATE_OF_ORIGIN.md

+Developer Certificate of Origin
+Version 1.1
+
+Copyright (C) 2004, 2006 The Linux Foundation and its contributors.
+1 Letterman Drive
+Suite D4700
+San Francisco, CA, 94129
+
+Everyone is permitted to copy and distribute verbatim copies of this
+license document, but changing it is not allowed.
+
+
+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.
\ No newline at end of file