Add DCO and note on how contributions are licensed

phpMyAdmin begins using a DCO as of 2013-12-01 to ensure changes are GPLv2-or-later.  Conservancy has drafted and approved the included Developer's Certificate of Origin ("DCO") and a related patch to the README to create a procedure whereby future contributors can use Signed-Off-By: in Git commits to confirm the patch is licensed under terms compatible with GPLv2-or-later.

Signed-Off-By: "Bradley M. Kuhn" <bkuhn@ebb.org>
Signed-Off-By: Tony Sebro <tony@sfconservancy.org>
Signed-Off-By: Denver Gingerich <denver@ossguy.com>
Merge remote branch 'upstream/master'
Fix 47e0223 contributor file patch submission text
Contributor file DCO text for 58f8106 per mynetx
Merge remote branch 'upstream/master'
Translated using Weblate (Ukrainian)

Currently translated at 94.2% (2584 of 2742)
Merge remote-tracking branch 'origin/QA_4_1' into QA_4_1
Display error message when using "Change Login Information / Copy User" dialog to update password (bug 4157)
Update po files
Add French translation for new error message.
Throw an error not to leave the interface.
Fix coding style
Merge pull request #771 from Tithugues/fix-4157-v2

Fix 4157: Display error message when using "Change Login Information / Copy User" dialog to update password
This is undefined when we copy a user without deleting the old one
Merge commit '38891e5'
Merge branch 'QA_4_1'
Fix typo
Translated using Weblate (Turkish)

Currently translated at 100.0% (2743 of 2743)
Translated using Weblate (Estonian)

Currently translated at 100.0% (2743 of 2743)
Fix coding style