Add Developer's Certificate of Origin

* CONTRIBUTING.md: Add 'Developer's Certificate of Origin 1.1' and
  decription how to 'sign-off' the commit.

Signed-off-by: Stefan Widgren <stefan.widgren@gmail.com>
Fix typo and line length in contributing guidelines

* CONTRIBUTING.md: Fix typo and update description of 'sync_libgit2'
  target in Makefile. Fix line lengths.

Signed-off-by: Stefan Widgren <stefan.widgren@gmail.com>