Add note about the SHARED_CORE requirement on some platforms also to the

INSTALL file because a lot of users don't read htdocs/manual/dso.html first.

PR: 2701


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/1.3.x@81815 13f79535-47bb-0310-9956-ffa450edef68