github: added CONTRIBUTING.md
README.md: make link a markdown link
Merge pull request #1064 from mehlis/CONTRIBUTING

github: added CONTRIBUTING.md
board: Initial import of the UDOO board

- fixed comment style and bracketing
- disabled newlib nano, added mac support
- fixed OSX compatibility in Makefile.include
examples/tests: blacklisted udoo board

blacklisted for:
- ccn-lite-client
- ccn-lite-relay
- rpl_udp
- test_net_if
- test_pnet