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>
Fix coding style
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Merge remote-tracking branch 'origin/master'
Fix bug#4146, set guid of wrapped function same as original function
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Merge remote branch 'upstream/master'
Merge branch 'QA_4_1'
Doc comment improvements
Fix 47e0223 contributor file patch submission text
Contributor file DCO text for 58f8106 per mynetx
Merge remote branch 'upstream/master'
Merge branch 'master' of github.com:phpmyadmin/phpmyadmin
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Merge remote-tracking branch 'origin/master'
Coding style fixes - valid class names
Merge branch 'master' of github.com:phpmyadmin/phpmyadmin
- Fix typo
- Fix coding style
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Merge remote-tracking branch 'origin/master'