Adds README

README.md

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+This repo is used to create a docker image containing arm64 binaries of `purs`
+(the purescript compiler) and `spago` (a build tool for purescript).
+
+The tag or branch you want to build must be set in their build scripts.
+
+Note that spago's has its own version number separate from purs so if you're
+upgrading then you must manually verfiy the version of purs and spago you want
+and make sure they work together. Generally you probably want the latest
+released version of each.
+
+This repository is typically used for simply providing these binaries inside of
+a docker image that can be used inside a project-specific docker image which
+needs an arm64 version.
+
+Once the build scripts have the appropriate versions in them then you should be
+able to produce the needed docker image with `docker build .`
+
+After that you can tag it or push it to whereever you need.
+
+Typically that will be `flipstone/purescript-arm64:<purs version>`
+
+e.g.
+
+```
+docker build . -t flipstone/purescript-arm64:0.15.4
+docker push
+```