Fix broken link

Correct link to LICENSES/ directory.

Signed-off-by: Gerwin Klein <gerwin.klein@data61.csiro.au>

github setup for CoC, License, Contributing

This commits adds links to the seL4 code of conduct (COC) and
contribution guidelines to the github setup. It also moves the license
remarks to its owns LICENSE.md file that is recognised by github.

Signed-off-by: Gerwin Klein <gerwin.klein@data61.csiro.au>