Update readme on build targets (#8)

Author: qiluo-msft

README.md

@@ -36,11 +36,13 @@ The root is disabled, but the created user could sudo.
 
 The target directory is ./target, containing the NOS installer image and docker images.
 - sonic-generic.bin: SONiC switch installer image (ONIE compatiable)
-- docker-base.gz: base docker image where others are built from (gzip tar archive)
+- sonic-aboot.bin: SONiC switch installer image (Aboot compatiable)
+- docker-base.gz: base docker image where other docker images are built from, only used in build process (gzip tar archive)
+- docker-database.gz: docker image for in-memory key-value store, used as inter-process communication (gzip tar archive)
 - docker-fpm.gz: docker image for quagga with fpm module enabled (gzip tar archive)
-- docker-orchagent.gz: docker image for SWitch State Service (SWSS)
-- docker-syncd.gz: docker image for the daemon to sync database and Broadcom switch ASIC
-- docker-syncd-mlnx.gz: docker image for the daemon to sync database and Mellanox switch ASIC
+- docker-orchagent.gz: docker image for SWitch State Service (SWSS) (gzip tar archive)
+- docker-syncd.gz: docker image for the daemon to sync database and Broadcom switch ASIC (gzip tar archive)
+- docker-syncd-mlnx.gz: docker image for the daemon to sync database and Mellanox switch ASIC (gzip tar archive)
 
 # Contribution guide