README improvements for community-default

fix header levels

default build is CentOS; RHEL product build is exceptional,
reorder accordingly

Signed-off-by: Jonathan Dowland <jdowland@redhat.com>

README.adoc

@@ -1,23 +1,24 @@
 # Source To Image (S2I) image for Red Hat OpenShift providing OpenJDK 1.8
 
-# Building
+This is the OpenShift Source to Image (S2I) image for OpenJDK 1.8, based
+on CentOS.
 
-## RHEL Based Image
+## Building
 
 ```
-cekit --overrides-file=product-overrides.yaml build
+cekit build
 ```
 
-This will produce an image named `redhat-openjdk-8/openjdk18-openshift`
+This will produce an image named `centos/openjdk-18-centos7`
 
-## CentOS Based Image
+### RHEL-Based Image
 
 ```
-cekit build
+cekit --overrides-file=product-overrides.yaml build
 ```
 
-This will produce an image named `centos/openjdk-18-centos7`
+This will produce an image named `redhat-openjdk-8/openjdk18-openshift`
 
-# License
+## License
 
 See link:LICENSE[LICENSE] file.