feat: add platforms doc

docs/platforms.md

@@ -1,3 +1,119 @@
 # Platforms
 
-TBD
+A platform is a React Native package that enables writing and shipping React Native applications to a new target. 
+
+For example, React Native Windows is a platform, because it allows to run React Native apps on Windows. At the same time, React Native itself is also a platform - it allows to run React Native apps on Android, iOS and tvOS.
+
+Each platform can have an additional configuration for the CLI to enable bundling apps and linking  and packages for targets it provides.
+
+## How does it work?
+
+A platform can define the following `react-native.config.js` at the root:
+```js
+const ios = require('@react-native-community/cli-platform-ios');
+const android = require('@react-native-community/cli-platform-android');
+
+module.exports = {
+  platforms: {
+    ios: {
+      linkConfig: ios.linkConfig,
+      projectConfig: ios.projectConfig,
+      dependencyConfig: ios.dependencyConfig
+    },
+    android: {
+      linkConfig: android.linkConfig,
+      projectConfig: android.projectConfig,
+      dependencyConfig: android.dependencyConfig
+    }
+  }
+}
+```
+> The above config adds support for linking Android and iOS dependencies by the CLI as well as bundling code for these platforms. This config can be found in [React Native repository](https://github.com/facebook/react-native/blob/0.60-stable/react-native.config.js) from 0.60 version on. 
+
+At the startup, React Native CLI reads configuration from all dependencies listed in `package.json` and reduces them into a single configuration. 
+
+At the end, a map of available platforms is passed to the bundler (Metro) to make it aware of the platforms available. This allows APIs such as `Platform.select` and requiring files with platform extensions to work properly.
+
+## Platform interface
+
+```ts
+type PlatformConfig<ProjectParams, ProjectConfig, DependencyConfig> = {
+  projectConfig: (string, ProjectParams) => ?ProjectConfig,
+  dependencyConfig: (string, ProjectParams) => ?DependencyConfig,
+  linkConfig: () => {
+    isInstalled: (ProjectConfig, string, DependencyConfig) => boolean,
+    register: (string, DependencyConfig, Object, ProjectConfig) => void,
+    unregister: (
+      string,
+      DependencyConfig,
+      ProjectConfig,
+      Array<DependencyConfig>,
+    ) => void,
+    copyAssets: (string[], ProjectConfig) => void,
+    unlinkAssets: (string[], ProjectConfig) => void,
+  },
+};
+```
+
+### projectConfig
+
+Returns a project configuration for a given platform. This is later used inside `linkConfig` to perform linking and unlinking.
+
+First argument is a root folder where the project is located. 
+
+Second argument is everything that users defined under:
+```js
+module.exports = {
+  project: {
+    [yourPlatformKey]: {}
+  }
+}
+```
+
+> Note: You may find this useful in order to alter the default behavior of your function. For example, on iOS, we find an `.xcodeproj` by globbing the project files and taking the first match. There's a possibility we pick the wrong one in case the project has multiple `.xcodeproj` files. In order to support this use-case, we have allowed users to define an exact path to an iOS project in order to overwrite our `glob` mechanism.
+
+We suggest performing all side-effects inside this function (such as resolving paths to native files) and making `linkConfig` functions pure, operating on provided data.
+
+### dependencyConfig
+
+Similar to [`projectConfig`](#projectconfig) above, but for a dependency of a project. 
+
+First argument is a path to a root folder of a dependency.
+
+Second argument is everything that dependency authors defined under:
+```js
+module.exports = {
+  dependency: {
+    [yourPlatformKey]: {}
+  }
+}
+```
+
+### linkConfig
+
+Returns an object with utilities that are run by the CLI while linking. 
+
+> Note: The following is deprecated and will stop working in the future. Consider providing a [`autolinking`](./autolinking.md) support.
+
+#### linkConfig.isInstalled
+
+Returns true if a library is already linked to a given project. False otherwise.
+
+#### linkConfig.register
+
+Performs platform-specific steps in order to link a library.
+
+#### linkConfig.unregister
+
+Performs platform-specific steps in order to unlink a library.
+
+#### linkConfig.copyAssets
+
+Performs platform-specific steps in order to copy assets of a library to a project.
+
+#### linkConfig.unlinkAssets
+
+Performs platform-specific steps in order to unlink assets of a library from a project.
+
+
+