pkg/tarsum: avoid buf2 allocation in Read

Docker-DCO-1.1-Signed-off-by: Cristian Staretu <cristian.staretu@gmail.com> (github: unclejack)
update the cli / man page docs based on what is in master right now

Docker-DCO-1.1-Signed-off-by: SvenDowideit <SvenDowideit@home.org.au> (github: SvenDowideit)
Add GoDoc and Travis build link/images to README

Signed-off-by: Vojtech Vitek (V-Teq) <vvitek@redhat.com>
Fix search test for busybox image

Signed-off-by: Michael Crosby <michael@docker.com>
Merge pull request #7628 from crosbymichael/fix-search-test

Fix search test for busybox image
Merge pull request #7604 from SvenDowideit/docs-update

update the cli / man page docs based on what is in master right now
Merge pull request #7613 from VojtechVitek/godoc_travis

Add GoDoc and Travis build link/images to README
Merge pull request #7602 from unclejack/tarsum_garbage_reduction

pkg/tarsum: avoid buf2 allocation in Read