Add fad-icon.png to spec.files (#283)

Attempt to fix broken image on https://www.rubydoc.info/gems/fastlane-plugin-firebase_app_distribution/

Author: kaibolay

fastlane-plugin-firebase_app_distribution.gemspec

@@ -14,7 +14,7 @@ Gem::Specification.new do |spec|
   spec.homepage      = "https://github.com/fastlane/fastlane-plugin-firebase_app_distribution"
   spec.license       = "MIT"
 
-  spec.files         = Dir["lib/**/*"] + %w(README.md LICENSE)
+  spec.files         = Dir["lib/**/*"] + %w(README.md LICENSE fad-icon.png)
   spec.test_files    = spec.files.grep(%r{^(test|spec|features)/})
   spec.require_paths = ['lib']