Adding DCO file and updating contributing.md with details

Author: Lee Mills

CONTRIBUTING.md

@@ -80,6 +80,22 @@ Also be sure to skim through our [ADRs](https://github.com/backstage/backstage/t
 
 If there are any updates in `markdown` file please make sure to run `yarn run lint:docs`. Though it is checked on `lint-staged`. It is required to install [vale](https://docs.errata.ai/vale/install) separately and make sure it is accessed by global command.
 
+## DCO
+
+As with other CNCF projects, Backstage has adopted a [Developers Certificate of Origin (DCO)](https://developercertificate.org/). A DCO is a lightweight way for a developer to certify that they wrote or otherwise have the right to submit code or documentation to a project.
+
+To certify the code you submit to the repository you'll need to add a `Signed-off-by` line to your commits.
+
+`$ git commit -s -m 'Awesome commit message'`
+
+Which will look something like the following in the repo;
+
+```
+Awesome commit message
+
+Signed-off-by: Jane Smith [email protected]
+```
+
 ## Creating Changesets
 
 We use [changesets](https://github.com/atlassian/changesets) to help us prepare releases. They help us make sure that every package affected by a change gets a proper version number and an entry in its `CHANGELOG.md`. To make the process of generating releases easy, it helps when contributors include changesets with their pull requests.

DCO

@@ -0,0 +1,37 @@
+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.