Updating the MapServer README...

[Steve Lime]

Hi all: I'm trying to spend more time getting us out of OSGeo incubation and am looking at all source files. One thing I noticed
in the README is that the Credits section is woefully out of date. There have been so many contributors that the section 
could get really long and still wouldn't be correct. Any objection to using the source files as means to identify contributors instead
and cutting the credits section out of the README? Most source files contain the name of the primary contributors already.

Steve