Add javadoc

Author: XePeleato

core/src/main/java/tech/tablesaw/columns/Column.java

@@ -164,6 +164,10 @@ default RollingColumn rolling(final int windowSize) {
     return new RollingColumn(this, windowSize);
   }
 
+  /**
+   * @param windowSize size of the rolling window to use
+   * @param center set the operation result at the center of the window or at the end of it
+   */
   default RollingColumn rolling(final int windowSize, final boolean center) {
     return new RollingColumn(this, windowSize, center);
   }