Move release notes README-*.adoc to separate directory
Also move two more release-related documents.
All referenced images are now in a common subdirectory.
Signed-off-by: Alexander Kriegisch <Alexander@Kriegisch.name>
Before, we used 1.9.7.BUILD-SNAPSHOT, which according to Andy Clement
was originally an intent across a group of Spring projects he was
involved in, to ensure that SNAPSHOTS were sorted alphabetically ahead
of MILESTONEs and ahead of RCs.
Signed-off-by: Alexander Kriegisch <Alexander@Kriegisch.name>