[Gradle] Add ReadMe.md to k.g.p-annotations module

^KT-54029 Verification Pending

libraries/tools/kotlin-gradle-plugin-annotations/ReadMe.md

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+## Gradle Plugin Annotations
+
+Contains all necessary annotations related to the Kotlin Gradle Plugin and modules around it. 
+This module has no dependencies and can therefore be considered very lightweight. 
+
+### API Surface Control Annotations
+
+#### InternalKotlinGradlePluginApi
+Can be used to mark APIs as 'internal' where an 'internal' modifier cannot be used
+(e.g. when entities need to be re-used across several modules within kotlin.git)
+
+
+#### ExperimentalKotlinGradlePluginApi
+Can be used to mark APIs as 'experimental'. No stability guarantees can be provided
+for APIs marked with this annotation