Mark npm issue in readme

Author: marijnh

README.md

@@ -4,6 +4,10 @@
 
 This is the central repository for [CodeMirror 6](https://codemirror.net/6). It holds the bug tracker and development scripts.
 
+If you want to **use** CodeMirror, install the separate packages from npm, and ignore the contents of this repository. If you want to **develop on** CodeMirror, this repository provides scripts to install and work with the various packages.
+
+**NOTE**: The npm workspaces setup used by this repository is broken with recent versions of npm, causing it to [fail](https://github.com/codemirror/codemirror.next/issues/710) when installing from scratch.
+
 To get started, make sure you are running [node.js](https://nodejs.org/) version 16. After cloning the repository, run
 
     node bin/cm.js install