Update the repository to reflect the requirement to sign the DCO

Going forward, we will be requiring contributors to sign the DCO. This
commit adds the DCO text itself and updates the contribution guidelines.

Signed-off-by: Martin Braun <[email protected]>

Author: mbr0wn

CONTRIBUTING.md

@@ -23,12 +23,22 @@ list, answers on Stack Overflow, Wiki page edits, examples... We very much
 appreciate those. However, this document is specifically about contributing
 code.
 
-## CLA filed?
+## DCO Signed?
 
-The GNU Radio codebase's copyright belongs to the Free Software Foundation. Any
-submission that gets merged must therefore be assigned to the FSF. Exceptions
-are small or obvious changes (< 10 lines, typically) or documentation changes.
-See also [What's this Copyright Assignment?][cla].
+Any code contributions going into GNU Radio will become part of a GPL-licensed,
+open source repository. It is therefore imperative that code submissions belong
+to the authors, and that submitters have the authority to merge that code into
+the public GNU Radio codebase.
+
+For that purpose, we use the [Developer's Certificate of Origin](DCO.txt). It
+is the same document used by other projects. Signing the DCO states that there
+are no legal reasons to not merge your code.
+
+To sign the DCO, suffix your git commits with a "Signed-off-by" line. When
+using the command line, you can use `git commit -s` to automatically add this
+line. If there were multiple authors of the code, or other types of
+stakeholders, make sure that all are listed, each with a separate Signed-off-by
+line.
 
 ## Coding Guidelines
 
@@ -79,6 +89,5 @@ please follow existing examples regarding their command line arguments, and
 reuse them.
 
 [grep1]: https://github.com/gnuradio/greps/blob/master/grep-0001-coding-guidelines.md
-[cla]: https://wiki.gnuradio.org/index.php/Development#What.27s_this_Copyright_Assignment.3F
 [wikicontrib]: https://wiki.gnuradio.org/index.php/Development
 [gr-devs]: https://github.com/orgs/gnuradio/teams/gr-devs

DCO.txt

@@ -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.