chore: Update README about maven migration (#1913)

* chore: Update README about maven migration

* chore: Update gradle references to maven in CONTRIBUTING.md

* chore: Update CONTRIBUTING.md

* chore: Update README.md

Author: lqiu96

CONTRIBUTING.md

@@ -24,13 +24,7 @@ use Github pull requests for this purpose.
 To build GAX:
 
 ```sh
-./gradlew build
-```
-
-To install GAX into the local maven repository:
-
-```sh
-./gradlew publishToMavenLocal
+mvn clean install
 ```
 
 ### The small print

README.md

@@ -20,6 +20,13 @@ a high risk of diamond dependency problems, because google-cloud-java uses beta
 library which can change in breaking ways between versions. See [VERSIONING](#versioning) for
 more information.
 
+> For new and existing Developers/ Contributors:
+> 
+> In December 2022, gax-java's build tool has been migrated from gradle to maven. 
+> Gradle related files are no longer being maintained and will be eventually removed.
+> 
+> The artifact coordinates in Maven Central (`{{ group_id }}:{{ artifact_id }}`) remain the same.
+
 Quickstart
 ----------