mention the cmake-based build

git-svn-id: http://svn.apache.org/repos/asf/httpd/httpd/trunk@1637112 13f79535-47bb-0310-9956-ffa450edef68
make docs

git-svn-id: http://svn.apache.org/repos/asf/httpd/httpd/trunk@1637113 13f79535-47bb-0310-9956-ffa450edef68