docs: create specification

Author: junhoyeo

README.md

@@ -0,0 +1,53 @@
+# 🛫 styled-platform
+
+## 📦 Installation
+
+```bash
+npm install styled-platform
+# Or with yarn
+yarn add styled-platform
+```
+
+## 🚀 Usage
+You can use this module in 3 different ways.
+
+```tsx
+import platform, { css } from 'styled-platform';
+
+const exampleText = styled.Text`
+  ${platform({
+    ios: css`
+      font-size: 14px;
+    `,
+    android: css`
+      font-size: 12px;
+    `,
+  })}
+`;
+```
+
+```tsx
+import platform, { css } from 'styled-platform';
+
+const exampleText = styled.Text`
+  font-size: 14px;
+
+  ${platform('android', css`
+    font-size: 14px;
+  `)}
+`;
+```
+
+```tsx
+import platform, { css } from 'styled-platform';
+
+const exampleText = styled.Text`
+  font-size: 14px;
+
+  ${platform.android(css`
+    font-size: 14px;
+  `)}
+`;
+```
+
+- The platform keys can be one of `'ios' | 'android' | 'macos' | 'windows' | 'web'`, which is from `PlatformOSType` of `react-native`([here](https://github.com/DefinitelyTyped/DefinitelyTyped/blob/master/types/react-native/index.d.ts#L5755)).