feat(QTDI-653): Immprove a readme of @lastgroup sample.

sample-parent/sample-features/aftergroup-lastgroup/README.md

@@ -14,4 +14,6 @@ java -jar sample-parent/sample-features/aftergroup-lastgroup/target/lastGroup-<v
 Now, methods with annotated with `@AfterGroup` in a `@Processor` connector, can have a boolean parameter annotated `@LastGroup`.  If this parameter is defined in the `@AfterGroup` method, it will be set to `true` for the last group, and only for it.
 
 # LastGroupProcessor Sample explanation
-This sample is a simple `@Processor` that uses the `@AfterGroup` annotation with a `@LastGroup` parameter, [as you can see here](./src/main/java/org/talend/sdk/component/feature/lastgroup/processor/LastGroupProcessor.java#L41).
+This sample is a simple `@Processor` that uses the `@AfterGroup` annotation with a `@LastGroup` parameter, [as you can see here](./src/main/java/org/talend/sdk/component/feature/lastgroup/processor/LastGroupProcessor.java#L41).
+
+This processor just buffers records, and, in the last call of `@AfterGroup` [method](./src/main/java/org/talend/sdk/component/feature/lastgroup/processor/LastGroupProcessor.java#L62), it will send all the records to the output. It also checks that the number of bufferized records is the same as the expected number set in its configuration [Config.java](./src/main/java/org/talend/sdk/component/feature/lastgroup/config/Config.java#L36).