[HUDI-249] Update Release-notes. Add sign-artifacts to POM and release related scripts. Add missing license headers

pom.xml

@@ -940,6 +940,20 @@
               <additionalparam>-Xdoclint:none</additionalparam>
             </configuration>
           </plugin>
+          <plugin>
+            <groupId>org.apache.maven.plugins</groupId>
+            <artifactId>maven-gpg-plugin</artifactId>
+            <version>1.4</version>
+            <executions>
+              <execution>
+                <id>sign-artifacts</id>
+                <phase>verify</phase>
+                <goals>
+                  <goal>sign</goal>
+                </goals>
+              </execution>
+            </executions>
+ 	  </plugin>
         </plugins>
       </build>
     </profile>