Use linux dco instead of our own contributors.txt (antlr#542)

* update doc

Signed-off-by: Terence Parr <[email protected]>

* use Linux DCO not contributors.txt

Signed-off-by: Terence Parr <[email protected]>

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,22 @@
+@@ -1,5 +1,23 @@
+<!--
+Thank you for proposing a contribution to the ANTLR jetbrains plugin!
+
+As of 1.18, we use 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 contributor's certificate of origin.)
+-->
+

developer-cert-of-origin.txt

@@ -0,0 +1,52 @@
+As of 1.18, the ANTLR plugin 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 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.