Add .gitignore to archetype-md-sal-app-simple

toolkit/main/archetypes/archetype-md-sal-app-simple/

should have a .gitignore that is similar to controller.

It should also exclude the

generate

project created, as that is a one time run thing, and thus shouldn't be configured.

The tricky part here is that currently the archetype includes 'generate' in the list of modules. So you need to figure out how to have the archetype created it,
but not include it in the parent pom.xml, and make sure folks can

cd generate/
mvn clean install -Dgen

and then fix the wiki instructions:

https://wiki.opendaylight.org/view/OpenDaylight_Toolkit:MD-SAL-Simple_Archetype