Add Spring logo for Spring posts

Via https://spring.io/img/spring-by-pivotal-9066b55828deb3c10e27e609af322c40.png

As it helps with better visibility on sharing metadata / displaying in
readers.
7 jobs for article/spring-extract-multipart-request-parameters in 7 minutes and 18 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #290210092
build_deploy_image

00:01:47

passed #290210089
build_site

00:01:36

passed #290210091
build_test_image

00:01:21

 
  Review
passed #290210093
review_deploy

00:00:47

passed #290210095
manual
review_stop

00:00:37

 
  Test
passed #290210096
test_image

00:01:59

passed #290210097
test_links

00:02:06