Add link to expat's site

Add commentary on why zlib is threadsafe (fully threadsafe as of 1.1.2
if default zalloc and zfree used - see zlib's changelog)


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@96409 13f79535-47bb-0310-9956-ffa450edef68