Import tool: document how to get an import module into an existing project

Following up on HIPPLUG-811.

We should document better how to set up the import tool when there is an existing (7.8) project.

There is some documentation at http://import-tool.forge.onehippo.org/internals.html but no step-by-step how to get an "import-admin" webapp as generated by the archetype, into existing project's code base.