Change documentation and plug-in file name

Add installation documentation.
The plug-in file name was renamed from LastModified.lrplugin to
LRLastModified.lrplugin. If you make an update now or in thenfuture, you have to
remove the old file manually.

1.1.0.0

Add "Cleanup Collection" checkbox. The default is "false", which means that existing photos will not be removed from target collection. This means also, the default behavior changed from 1.0.0.0