Update required npm version in README

package-lock.json was updated to v2 in b1ecde6

Author: BartSchuurmans

README.md

@@ -18,7 +18,7 @@ This is the official documentation platform for the [ReScript](https://rescript-
 ## System Requirements
 
 - `[email protected]` or higher (for ES6 module compat)
-- `npm@6` (package-lock v1)
+- `npm@7` (package-lock v2)
 
 ## Setup