docs: Improve usage guidance

README.md

@@ -9,7 +9,15 @@ This repo holds Nix flake, modules, packages, and reusable utility Nix language
 Currently, you can build a package directly like so:
 
 ```bash
-nix build github.com:OpenGen/nix#ociImgBase
+nix build github.com:OpenGen/nix#sppl
+```
+
+### Build an OCI image with an environment
+
+OCI images consume these libraries and ones from other OpenGen repos, and are specified in another flake (excepting the `base` oci image):
+
+```bash
+nix build github.com:OpenGen/nix#ociImgLoom
 ```
 
 ### Import utility code
@@ -20,13 +28,45 @@ To access the `lib` code exported by this flake, declare this repo as a flake in
 {
   inputs = {
     nixpkgs.url = ...
-    gensqlnix.url = "github:OpenGen/nix";
+    opengen.url = "github:OpenGen/nix";
   };
-  outputs = inputs@{ nixpkgs, gensqlnix, ... }: let
+  outputs = inputs@{ nixpkgs, opengen, ... }: let
     # call some function
-    toolbox = gensqlnix.lib.basicTools "aarch64-darwin";
+    toolbox = opengen.lib.basicTools "aarch64-darwin";
   in {
     ...
   };
 };
 ```
+
+## Packages
+
+### `.#sppl`
+
+Python [library by ProbSys](https://github.com/probsys/sppl) packaged for python3.9 .
+
+### `.#loom`
+
+Implementation of [CrossCat in Python](https://github.com/posterior/loom). NOTE: this ONLY builds for `x86_64` architectures and only runs on linux, because it depends on
+platform-dependent `distributions`.
+
+Your options are:
+
+```bash
+nix build '.#packages.x86_64-linux.loom'                      # same as `.#loom` if that is your OS/arch
+nix build './envs-flake#packages.x86_64-darwin.ociImgLoom'
+```
+
+If you are running on Mac silicon (`aarch64-darwin`), that OCI image will run but behavior is not defined or supported.
+
+#### `.#loom.morePackages.distributions`
+
+Native library for probability distributions in python used by Loom. NOTE: this ONLY builds for `x86_64` architectures and only runs on linux.
+
+#### `.#loom.morePackages.parsable`
+#### `.#loom.morePackages.pymetis`
+#### `.#loom.morePackages.goftests`
+#### `.#loom.morePackages.tcmalloc`
+
+Other upstream python packages required by Distributions and/or Loom.
+