So, a lot of people still try to get the Jar-in-Jar system in their mod working and it seems there is no documentation available. This wont be a guide (its most likely not the optimal way to do it), but it might help you to get started.   Forge allows you to specify the ContainedDep manifest attribute. This attribute is a space separated list of file names (jar files) which will be extracted from your jar and then loaded into the mod-class loader (at the time of writing, those mods are