Add a note about expo env

Author: brentvatne

README.md

@@ -1,5 +1,7 @@
 # react-navigation-addon-search-layout
 
-More documentation coming..
+Note: this currently depends on the Expo environment being present. I
+may fix this in the future, a PR is welcome though.  More documentation
+coming..
 
 [Try it on Snack](https://snack.expo.io/HJXlZ7TuW)