Document the NUnit bug that causes tests to fail on non-Windows platforms with a "Too many open files" exception.
Document the NUnit bug that causes tests to fail on non-Windows platforms with
a "Too many open files" exception.

BenJenkinson/nodatime,jskeet/nodatime,nodatime/nodatime,malcolmr/nodatime,zaccharles/nodatime,malcolmr/nodatime,malcolmr/nodatime,malcolmr/nodatime,zaccharles/nodatime,jskeet/nodatime,zaccharles/nodatime,zaccharles/nodatime,zaccharles/nodatime,nodatime/nodatime,BenJenkinson/nodatime,zaccharles/nodatime