step description update (#16)

* step description update

* updating file based on review comment

step.yml

@@ -1,6 +1,44 @@
 title: Flutter Build
-summary: This step will build a Flutter project
-description: This step will build a Flutter project
+summary: Builds a Flutter project.
+description: |-
+  This Step builds an iOS and an Android app. By default the Step builds the artifact based on the platform the scanner detects.
+  
+  ### Configuring the Step
+  1. In the **Project Location** input the root directory of your Flutter project is automatically filled out.
+  2. Select which platform your project should be built for (`ios`, `android` or `both`).
+  3. Enable **Debug** option to get verbose logs and see where the Step is failing.
+  
+  Depending on the selected platform/s, continue with the rest of the config inputs.
+
+  #### Configuring for an iOS app
+  1. Make sure the **Platform input** is set to `iOS` or `both`.
+  2. In **Codesign Identity** you can onverride the code signing identities that you set in Flutter.
+  3. In **Additional parameters** add any flag to customize your build (for example, the `--release` flag appended to `flutter build io` builds a deployable iOS app).
+  4. Leave the **Output pattern** input's default value as is or modify it to the pattern if your build artifacts are stored elsewhere.
+  5. Make sure you have the **Xcode Archive & Export for iOS** Step after the **Flutter Build** Step in your Workflow.
+
+  #### Configuring for an Android app
+  1. Insert the **Android Sign** Step after the **Flutter Build** Step and make sure code signing files are uploaded to the **Code Signing** tab.
+  2. Make sure the **Platform input** is set to `Android` or `both`.
+  3. Scroll down to the `Android Platform Configs` input section, and select the preferred output artifact type you wish to generate in the **Android output artifact type** input. The Step can build an APK and an Android App Bundle as well.
+  4. Append any flag to the `build` command in the **Additional parameters** input.
+  5. Leave the **Output pattern** input's default value as is or modify it to the pattern if your build artifacts are stored elsewhere.
+
+  ### Troubleshooting
+
+  Make sure the **Flutter Install** Step is before the **Flutter Build** Step.
+  If you have not set up code signing correctly, some code signing related issue will definitely surface by this build Step. 
+  If you're unsure about code signing, consult our guide linked in Useful links.
+
+  ### Useful links
+  - [Getting started with Flutter apps](https://devcenter.bitrise.io/getting-started/getting-started-with-flutter-apps/#deploying-a-flutter-app)
+  - [Available version tags](https://github.com/flutter/flutter/releases)
+  - [Available branches](https://github.com/flutter/flutter/branches)
+  - [Code signing](https://devcenter.bitrise.io/code-signing/code-signing-index/)
+
+  ### Related Steps
+  - [Flutter Install](https://www.bitrise.io/integrations/steps/flutter-installer)
+  - [Flutter Test](https://www.bitrise.io/integrations/steps/flutter-test)
 website: https://github.com/bitrise-steplib/bitrise-step-flutter-build
 source_code_url: https://github.com/bitrise-steplib/bitrise-step-flutter-build
 support_url: https://github.com/bitrise-steplib/bitrise-step-flutter-build/issues
@@ -141,4 +179,4 @@ outputs:
         This output will include the path of the generated AAB file,
         after filtering based on the filter inputs.
         If the build generates more than one AAB file which fulfills the
-        filter inputs this output will contain the last one's path.
+        filter inputs this output will contain the last one's path.