README start page: improve developement procedures

Hi,
regarding our virtual  meeting yesterday, I tried to improve the RIOT start page and the wiki a bit to prevent overlaps between the work of multiple developers. Let me present the changes in a nutshell so we can discuss about it:

README:
- In section "CONTRIBUTE" there have been eight bullet points which equalled 11 bullet points in the "Developement procedures" wiki. I would vote for either put all points on the start page or just give a linkt to the "Developement procedures" site. I realised the second alternative.
- What du you think?

Developement procedures: 
- I added point 11. (wasn't shure where else it is appropriate) which tells to create a wiki page, open and link an issue and put this on the mailing list once when porting for new boards.
https://github.com/RIOT-OS/RIOT/wiki/Development-procedures

wiki/Board:-Samr21-xpro:
- Under "Implementation Status" I linked the tracker
https://github.com/RIOT-OS/RIOT/wiki/Board:-Samr21-xpro