Add presubmit workflow badge to the README (#90)

Author: pq

README.md

@@ -1,5 +1,7 @@
 # Dart IntelliJ Plugin
 
+[![presubmit](https://github.com/flutter/dart-intellij-third-party/actions/workflows/presubmit.yaml/badge.svg)](https://github.com/flutter/dart-intellij-third-party/actions/workflows/presubmit.yaml)
+
 ## Development setup
 
 1. Clone this repository with
@@ -99,4 +101,4 @@ Before the move to this repository, the project had 4 working and active branche
 - `252` - 2025.2
 - `master` - the current master for intellij-community, targeting the next version 2025.3
 
-In its original location the project was built with Bazel; however, in github.com/flutter, the project is built using Gradle (see https://plugins.jetbrains.com/docs/intellij/tools-intellij-platform-gradle-plugin.html).
+In its original location the project was built with Bazel; however, in github.com/flutter, the project is built using Gradle (see https://plugins.jetbrains.com/docs/intellij/tools-intellij-platform-gradle-plugin.html).