Add a section on writing transformation connectors.  Part of CONNECTORS-959

git-svn-id: https://svn.apache.org/repos/asf/manifoldcf/trunk@1601605 13f79535-47bb-0310-9956-ffa450edef68
Update concepts document.  Part of CONNECTORS-959.

git-svn-id: https://svn.apache.org/repos/asf/manifoldcf/trunk@1601608 13f79535-47bb-0310-9956-ffa450edef68