docs: add README

Author: TheButlah

README.md

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+# UpmBuilds
+
+This repo provides scripts and CI for building various Unity Package Manager
+packages. 
+
+## What this produces
+
+Fundamentally, the output of this repo is files like `org.basisvr.simplebase-4.0.2.tgz`.
+These are basically gzipped tar files (tar.gz files, kinda like zip files) that
+the Unity Package Manager can consume as a package.
+
+They are added to unity via the tarball import mechanism, read more 
+[here](https://docs.unity3d.com/6000.0/Documentation/Manual/upm-ui-tarball.html).
+
+## How to build
+
+CI builds these automatically on every merge to main, or you can manually
+dispatch the workflow by clicking "Run Workflow" 
+[here](https://github.com/BasisVR/UpmBuilds/actions/workflows/ci.yaml), if you
+have write access to the repo. This results in a github artifact that gets
+uploaded to the CI run, which you can download.
+
+You can also clone the repo, install [just](https://just.systems), and then run 
+`just all`. This is similar to a makefile and it will build all of the code.
+
+The resulting artifacts live in the `build` directory.