docs: add note for windows issues

Closes #192

Author: Kent C. Dodds

README.md

@@ -137,6 +137,13 @@ contains special shell characters *that you want interpreted*, then use
 
 On Windows you need to use `cross-env-shell`, if you want to handle [signal events](https://nodejs.org/api/process.html#process_signal_events) inside of your program. A common case for that is when you want to capture a `SIGINT` event invoked by pressing `Ctrl + C` on the command-line interface.
 
+## Windows Issues
+
+Please note that `npm` uses `cmd` by default and that doesn't support command
+substitution, so if you want to leaverage that, then you need to update your
+`.npmrc` to set the `script-shell` to powershell.
+[Learn more here](https://github.com/kentcdodds/cross-env/issues/192#issuecomment-513341729).
+
 ## Inspiration
 
 I originally created this to solve a problem I was having with my npm scripts in