chore(layers): mark styled-components as external dependencies (#253)

* Mark styled-components as external dependencies

* Add installing styled-components to layers Readme

Author: akingbadeayobami

packages/layers/README.md

@@ -27,7 +27,7 @@
 ## Usage
 
 ```bash
-yarn add @craftjs/layers
+yarn add @craftjs/layers styled-components
 ```
 
 ```jsx

packages/layers/package.json

@@ -31,15 +31,16 @@
   },
   "homepage": "https://github.com/prevwong/craft.js/",
   "dependencies": {
-    "@craftjs/utils": "^0.1.0-beta.19",
-    "styled-components": "^4.2.1"
+    "@craftjs/utils": "^0.1.0-beta.19"
   },
   "devDependencies": {
     "@babel/core": "^7.7.4",
-    "@svgr/rollup": "^4.3.3"
+    "@svgr/rollup": "^4.3.3",
+    "styled-components": "^4.2.1"
   },
   "peerDependencies": {
     "@craftjs/core": "^0.1.0-alpha.1",
-    "react": "^16.8.0"
+    "react": "^16.8.0",
+    "styled-components": ">= 4"
   }
 }