SONARGO-109 Update README.md: describe how to use this project as a git submodule (#16)

Co-authored-by: Jonas Wielage <jonas.wielage@sonarsource.com>

Author: petertrr

README.md

@@ -1,3 +1,36 @@
 # cloud-native-gradle-modules
 
 Common Gradle modules for multiple projects
+
+# Usage in a project
+This repository is supposed to be used as a submodule in a project. The Gradle build should then be configured as an include build.
+
+To include this repository as a submodule in a project, run the following command in the project's root directory:
+```bash
+git submodule add https://github.com/SonarSource/cloud-native-gradle-modules gradle/build-logic
+```
+
+Then, in the project's `settings.gradle.kts` file, add `includeBuild` to the `pluginManagement` block:
+```kotlin
+pluginManagement {
+    includeBuild("gradle/build-logic")
+}
+```
+
+Here we used `gradle/build-logic` as a directory, but the choice is arbitrary.
+An example usage can be seen in the [Sonar-Go analyzer](https://github.com/SonarSource/sonar-go/blob/d4b923d43c3183927a32dc0956cbf4e4ec50d8a9/settings.gradle.kts#L17)
+
+### Configure Git to Automatically Checkout Changes in the Submodule
+
+When a newer version of the submodule is integrated into a remote branch, running `git pull` will not automatically update the submodule. Instead, Git will display it as changed, and `git status` will show a message like:
+```text
+modified:   gradle/build-logic (new commits)
+```
+
+To configure Git to automatically checkout changes in the submodule, run the following command:
+
+```bash
+git config submodule.recurse true
+```
+
+Optionally, run this command with `--global` to apply this configuration globally.