Revert "Remove mention of psc-package (#272)" (#277)

This reverts commit 25aa94a.

Author: hdgarrood

ecosystem/Editor-and-tool-support.md

@@ -38,6 +38,7 @@ Spacemacs users can just use the [`purescript` layer](https://github.com/syl20bn
 ## Build tools and package managers
 
 - [Pulp](https://github.com/purescript-contrib/pulp) - a standalone build system for PureScript ([pulp](https://www.npmjs.com/package/pulp) in `npm`)
+- [psc-package](https://github.com/purescript/psc-package) - A package manager for PureScript based on the concept of package sets
 - [spago](https://github.com/spacchetti/spago) - PureScript package manager and build tool powered by [Dhall](https://github.com/dhall-lang/dhall-lang) and [package-sets](https://github.com/purescript/package-sets)
 - [Gulp task](https://github.com/purescript-contrib/gulp-purescript) (`gulp-purescript` in `npm`)
 - [psvm-js](https://github.com/ThomasCrvsr/psvm-js) - PureScript Version Manager