update readme

Author: MattDiMu

README.md

@@ -37,6 +37,11 @@
 ### Installation
 `npm install --save-dev postcss postcss-replace-overflow-wrap`
 
+For Postcss 7 or earlier use Version 3 or earlier:
+
+`npm install --save-dev postcss-replace-overflow-wrap@3`
+
+
 ## Usage
 
 ```js