Set date-fns as a peerDependency

Author: onurkerimov

package.json

@@ -45,7 +45,8 @@
     "react-list": "^0.8.13"
   },
   "peerDependencies": {
-    "react": "^0.14 || ^15.0.0-rc || >=15.0"
+    "react": "^0.14 || ^15.0.0-rc || >=15.0",
+    "date-fns": "2.0.0-alpha.7 || >=2.0.0"
   },
   "devDependencies": {
     "@babel/cli": "^7.7.7",