Add note for optional styled-components dependency

Fix #13

Author: kitten

README.md

@@ -137,6 +137,24 @@ already, `ssrPrepass` will suspend on `<Query>` components.
 
 [More information can be found in Apollo's own docs](https://www.apollographql.com/docs/react/features/server-side-rendering.html#getDataFromTree)
 
+## Optional Dependencies
+
+### Special case for `styled-components`
+
+`react-ssr-prepass` has an optional dependency on `styled-components@>=4.0.0`.
+This optimization skips any styling logic and imitates the attributes and props
+logic of `styled-components` otherwise.
+
+It exists because styled-components may populate the `ServerStyleSheet` as
+part of a `react-ssr-prepass` run, since it behaves like a normal React renderer.
+This is dangerous if the element tree that is passed to `react-ssr-prepass`
+is not wrapped in a `ServerStyleSheet` at all, since it will start accumulating
+styles and cause a memory leak. In `v4` it can also cause these styles to
+become _global_ and be sent to every server-rendered page.
+
+If you're not using styled-components however and are bundling your server-side
+code you need to stub styled-components with an empty package or just install it.
+
 ## Prior Art
 
 This library is (luckily) not a reimplementation from scratch of

package.json

@@ -60,6 +60,9 @@
     "react": "^16.8.0",
     "react-is": "^16.8.0"
   },
+  "dependencies": {
+    "object-is": "^1.0.1"
+  },
   "devDependencies": {
     "@babel/core": "^7.3.4",
     "@babel/preset-env": "^7.3.4",
@@ -77,8 +80,5 @@
     "react-dom": "^16.8.4",
     "react-is": "^16.8.4",
     "styled-components": "^4.2.0"
-  },
-  "dependencies": {
-    "object-is": "^1.0.1"
   }
 }