Resolve "Update Contributor's Guide to reflect gitlab and new procedures"

Closes #50 (closed) This is somewhat more up to date. I copied the 'steps' included in the Kea contributor's guide. I linked the DHCP Developer's Survival guide in the wiki (at least I linked to the wiki). I made a lot of small improvements in the grammar, which are probably unnecessary but I couldn't help it.

The contributions guidance we had on the old website covered the topics of licensing, copyrights, and coding standards as well. I added one sentence about licensing at the top, which might be inadequate. ?? I have a section on implementing that is blank - I thought we had some coding standards somewhere, but I couldn't find them in Gitlab. I put it in there in case there are some coding standards we can include.

I considered and then deleted something about how submissions for the /contrib directory could be separately licensed. I am not sure I know the 'rules' for /contrib, but I think it is probably easier to accept contributions that are destined for that directory. Ditto for Man page corrections and improvements.

I did link to the Gitlab How To I found in the wiki, but it kind of overlaps with this contributors guide. NB there is a whole developers section under /docs that if full of what looks to me like old junk. I didn't dare suggest deleting any of it, but I also wouldn't want to recommend anyone read it.