add to README

[cole-brokamp]

you shouldn't need to specify versions of dependencies

Looking at a DESCRIPTION file should tell you what packages the code depends on. Looking at an renv file, for example, does not let you quickly tell which packages you have declared and which packages are dependencies of your project package dependencies

• we will assume that a given version of a package works with the versions of its dependencies as stated in its own DESCRIPTION file
• if an older version of a package has problems using a newer version of one of its dependencies, then "pin" the version of that package dependency by declaring it in a dummy R file somewhere in the project (i.e., packages.R)