Update readme for using next tag with yarn

gauravtiwari · web-flow · commit 26e5a5f18133 · 2018-10-01T08:55:17.000+01:00
diff --git a/README.md b/README.md
@@ -91,7 +91,7 @@ yarn add https://github.com/rails/webpacker.git
 
 # OR to try out 4.x pre-release
 gem 'webpacker', '>= 4.0.x'
-yarn add @rails/webpacker@4.0.0-pre.2
+yarn add @rails/webpacker@next
 ```
 
 Finally, run the following to install Webpacker:
