Some documentation

docs/platforms.md

@@ -40,6 +40,7 @@ At the end, a map of available platforms is passed to the bundler (Metro) to mak
 
 ```ts
 type PlatformConfig<ProjectParams, ProjectConfig, DependencyConfig> = {
+  npmPackageName?: string;
   projectConfig: (string, ProjectParams) => ?ProjectConfig,
   dependencyConfig: (string, ProjectParams) => ?DependencyConfig,
   linkConfig: () => {
@@ -57,6 +58,13 @@ type PlatformConfig<ProjectParams, ProjectConfig, DependencyConfig> = {
 };
 ```
 
+### npmPackageName
+
+Returns the name of the npm package that should be used as the source for react-native JS code for platforms that provide platform specific overrides to core JS files.  This causes the default metro config to redirect imports of react-native to another package based when bundling for that platform.  The package specified should provide a complete react-native implementation for that platform.
+
+If this property is not specified, it is assumed that the code in core `react-native` works for the platform.
+
+
 ### projectConfig
 
 Returns a project configuration for a given platform or `null`, when no project found. This is later used inside `linkConfig` to perform linking and unlinking.