Additional documentation (flutter-mapbox-gl#986)

* Added docs for running example code on iOS

* Updated running example code docs
Resized screenshot
Added index to Readme

README.md

@@ -4,6 +4,30 @@
 > 
 > We welcome [feedback](https://github.com/tobrun/flutter-mapbox-gl/issues) and contributions.
 
+
+## Table of contents
+
+- [Flutter Mapbox GL](#flutter-mapbox-gl)
+  - [Table of contents](#table-of-contents)
+  - [Introduction](#introduction)
+  - [Setting up](#setting-up)
+    - [Mobile](#mobile)
+      - [Secret Mapbox access token](#secret-mapbox-access-token)
+    - [Web](#web)
+    - [All platforms](#all-platforms)
+      - [Public Mapbox access token](#public-mapbox-access-token)
+  - [Supported API](#supported-api)
+  - [Map Styles](#map-styles)
+  - [Offline Sideloading](#offline-sideloading)
+  - [Downloading Offline Regions](#downloading-offline-regions)
+  - [Location features](#location-features)
+    - [Android](#android)
+    - [iOS](#ios)
+  - [Running the example code](#running-the-example-code)
+  - [Contributing](#contributing)
+
+## Introduction
+
 This Flutter plugin allows to show embedded interactive and customizable vector maps inside a Flutter widget. For the Android and iOS integration, we use [mapbox-gl-native](https://github.com/mapbox/mapbox-gl-native). For web, we rely on [mapbox-gl-js](https://github.com/mapbox/mapbox-gl-js). This project only supports a subset of the API exposed by these libraries. 
 
 ![screenshot.png](screenshot.png)
@@ -200,6 +224,8 @@ xml ...
 
 [Recommended](https://docs.mapbox.com/help/tutorials/first-steps-ios-sdk/#display-the-users-location)  explanation about "Shows your location on the map and helps improve the map".
 
+## Running the example code
+See the [documentation about this topic](doc/RUNNING_EXAMPLE_CODE.md)
 
 ## Contributing
 

doc/RUNNING_EXAMPLE_CODE.md

@@ -0,0 +1,58 @@
+# Running the example code
+
+- You'll find the example code in the `example` folder of this repository
+
+## Secret Mapbox access token
+
+A secret access token with the `Downloads: Read` scope is required for the underlying Mapbox SDKs to be downloaded.
+Information on setting it up is available in the Mapbox documentation:
+[Android](https://docs.mapbox.com/android/maps/guides/install/),
+[iOS](https://docs.mapbox.com/ios/maps/guides/install/).
+
+If the properly configured token is not present,
+the build process fails with one the following errors *(for Android/iOS respectively)*:
+
+```
+* What went wrong:
+A problem occurred evaluating project ':mapbox_gl'.
+> SDK Registry token is null. See README.md for more information.
+```
+
+```
+[!] Error installing Mapbox-iOS-SDK
+curl: (22) The requested URL returned error: 401 Unauthorized
+```
+
+## iOS
+- Clone this repo
+- Open `example/ios/Runner.xcworkspace` in XCode
+- On the left, select Runner and change the Bundle identifier on the `general` tab to something of your own.
+
+![XCode bundle identier](img/xcode-bundle-identifier.png)
+
+- On the `signing & capabilities` tab change the team to your own.
+- Close Xcode
+- run `flutter build ios --dart-define ACCESS_TOKEN=sk.xxxxxxxxxxxxxxxxxxxxx` to make sure the project builds from the command line. The `ACCESS_TOKEN` is the Mapbox token you've obtained in the `Secret Mapbox access token` step described above.
+- The build command output should be something like:
+
+```terminal
+Warning: You are using these overridden dependencies:
+! mapbox_gl_platform_interface 0.15.0 from path ../mapbox_gl_platform_interface
+! mapbox_gl_web 0.15.0 from path ../mapbox_gl_web
+Running "flutter pub get" in example...                          1,918ms
+Building com.xxx.mapboxtest for device (ios-release)...
+Updating project for Xcode compatibility.
+Upgrading project.pbxproj
+Upgrading Runner.xcscheme
+Signing iOS app for device deployment using developer identity: "Apple Development: xxx@xxxxx.xx (XXXXXXX)"
+Running pod install...                                           2,950ms
+Running Xcode build...
+ └─Compiling, linking and signing...                        15.1s
+Xcode build done.                                           98.2s
+Built /Development/flutter/maps/example/build/ios/iphoneos/Runner.app.
+```
+
+- Open `example/ios/Runner.xcworkspace` again in XCode 
+- Run the project connected iPhone or iOS Emulator
+
+![XCode bundle identief](img/example-iphone.jpeg)