THRIFT-5570: update java lib document about gradle usage (#2583)

* update java doc about gradle usage
* update .gitignore
diff --git a/build/cmake/FindGradle.cmake b/build/cmake/FindGradle.cmake
index 8845d69..3806bd8 100644
--- a/build/cmake/FindGradle.cmake
+++ b/build/cmake/FindGradle.cmake
@@ -21,7 +21,7 @@
 #  GRADLE_FOUND - system has Gradle
 #  GRADLE_EXECUTABLE - the Gradle executable
 #
-# It will search the environment variable ANT_HOME if it is set
+# It will search the environment variable GRADLE_HOME if it is set
 
 include(FindPackageHandleStandardArgs)