The package manager in this case is "opkg". Additionally, in the following thread you will find a very useful discussion about a repo for Edison: Edison opkg package repo created. You will find the link to the repo with some instructions to use it. Hopefully the packages you want to use are already available in the repo, but if not you could post a request in the thread for the package you want use.
Regarding the documentation on how to make custom images, I will recommend you this document: Intel® Edison Products — Board Support Package (BSP) User Guide