Clarify versions in README (#165)

Author: Danny Tatom

README.md

@@ -17,11 +17,13 @@ Status: Alpha
 
 ```bash
 # npm
-npm install reactfire
+npm install reactfire@next
 # yarn
-yarn add reactfire
+yarn add reactfire@next
 ```
 
+If you like living life on the edge, use `reactfire@canary`.
+
 ## Example use
 
 Check out the [live version on StackBlitz](https://stackblitz.com/edit/reactfire-sample)!