Adding contribution guidelines to user guide, including new Developer's Certificate of Origin
diff --git a/DCO.txt b/DCO.txt
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+++ b/DCO.txt
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+Developer's Certificate of Origin 1.1
+
+By making a contribution to this project, I certify that:
+
+(1) 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
+
+(2) 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
+
+(3) The contribution was provided directly to me by some other
+ person who certified (1), (2) or (3) and I have not modified
+ it.
+
+(4) 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.