Closes#1880
As a recent first-time contributor to puma, there are some of the things
that might have been helpful to have in one place as I worked on my
first changes.
I am also thinking about a resources section that links to the docs
directory and to docs for libraries that might be unfamiliar to someone
looking through puma for the first time (I didn't know much about
TCPSocket, IO.pipe, nio4r, for example)