readme documentation about currently supported Spago (#38)

* readme documentation about currently supported Spago

* Update README.md

Co-authored-by: Thomas Honeyman <[email protected]>

---------

Co-authored-by: Thomas Honeyman <[email protected]>

Author: peterbecich

README.md

@@ -7,6 +7,7 @@ A GitHub Action which sets up a PureScript toolchain for CI. Contains the follow
 
 - The [PureScript compiler](https://github.com/purescript/purescript)
 - The [Spago package manager and build tool](https://github.com/purescript/spago)
+  - Use the `unstable` version for spago-next; otherwise, setup-purescript provides [`spago-legacy`](https://github.com/purescript/spago-legacy); see [issue 37](https://github.com/purescript-contrib/setup-purescript/issues/37)
 - The [`psa` error reporting frontend for the compiler](https://github.com/natefaubion/purescript-psa)
 
 You can also optionally include the following tools: