Use linux DCO not our old contributors certificate of origin

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,5 +1,23 @@
 <!--
-Thank you for proposing a contribution to the ANTLR project. In order to accept changes from the outside world, all contributors must "sign" the  [contributors.txt](https://github.com/antlr/antlr4/blob/master/contributors.txt) contributors certificate of origin. It's an unfortunate reality of today's fuzzy and bizarre world of open-source ownership.
+Thank you for proposing a contribution to the ANTLR project!
 
-Make sure you are already in the contributors.txt file or add a commit to this pull request with the appropriate change. Thanks!
--->
+(Please make sure your PR is in a branch other than dev or master
+ and also make sure that you derive this branch from dev.)
+
+As of 4.10, ANTLR uses the Linux Foundation's Developer
+Certificate of Origin, DCO, version 1.1. See either
+https://developercertificate.org/ or file 
+contributors-cert-of-origin.txt in the main directory.
+
+Each commit requires a "signature", which is simple as
+using `-s` (not `-S`) to the git commit command: 
+
+git commit -s -m 'This is my commit message'
+
+Github's pull request process enforces the sig and gives
+instructions on how to fix any commits that lack the sig.
+See https://github.com/apps/dco for more info.
+
+No signature is required in this file (unlike the
+previous ANTLR contributor's certificate of origin.)
+-->

developer-cert-of-origin.txt

@@ -0,0 +1,52 @@
+As of 4.10, ANTLR uses the Linux Foundation's Developer
+Certificate of Origin, DCO, version 1.1. See either
+https://developercertificate.org/ or the text below.
+
+Each commit requires a "signature", which is simple as
+using `-s` (not `-S`) to the git commit command: 
+
+git commit -s -m 'This is my commit message'
+
+Github's pull request process enforces the sig and gives
+instructions on how to fix any commits that lack the sig.
+See https://github.com/apps/dco for more info.
+
+No signature is required in this file (unlike the
+previous ANTLR contributor's certificate of origin.)
+
+----- https://developercertificate.org/ ------
+
+Developer Certificate of Origin
+Version 1.1
+
+Copyright (C) 2004, 2006 The Linux Foundation and its contributors.
+
+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.

doc/releasing-antlr.md

@@ -4,6 +4,14 @@
 
 Create a pre-release or full release at github; [Example 4.5-rc-1](https://github.com/antlr/antlr4/releases/tag/4.5-rc-1).
 
+### Turn on DCO Enforcement
+
+As of 4.10, we will be using the Linux DCO not the previous contributors license agreement that required signing the file. Now, we use the DCO and contributors must use `-s` on each commit to the branch associated with a pull request.
+
+See [GitHub App DCO](https://github.com/apps/dco).
+
+Make sure this has turned on for the `dev` and `master` branch.
+
 ### Delete existing release tag
 
 Wack any existing tag as mvn will create one and it fails if already there.