INSTALL.md needs a refresh

[heather999]

I'm looking to install the shiny new DC2 release https://github.com/LSSTDESC/Twinkles/releases/tag/DC2-Run1.1-v3. After downloading the tarball, looking at INSTALL.md, minor bits like the conda install of the stack need to be updated, but more importantly, I want to know what to do after I have the stack and the python requirements, and have the stack set up (installed without lsstsw). Would a user run one of the scripts in the setup directory, every time they want to use Twinkles?