Doc: README and INSTALL update

Minor cleanups, updates and clarifications. Also removes (incomplete
and well-known) build steps from README, as they are better described
in INSTALL.
32 jobs from int-new in 4 minutes and 41 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Image
passed #214405
dind allowed to fail
docker_centos-6-amd64

00:00:32

passed #214406
dind allowed to fail
docker_centos-7-amd64

00:00:29

passed #214395
dind allowed to fail
docker_debian-7-amd64

00:00:25

passed #214399
dind allowed to fail
docker_debian-7-i386

00:00:31

passed #214396
dind allowed to fail
docker_debian-8-amd64

00:00:31

passed #214400
dind allowed to fail
docker_debian-8-i386

00:00:32

passed #214397
dind allowed to fail
docker_debian-9-amd64

00:00:31

passed #214401
dind allowed to fail
docker_debian-9-i386

00:00:32

passed #214398
dind allowed to fail
docker_debian-testing-amd64

00:00:31

passed #214402
dind allowed to fail
docker_debian-testing-i386

00:00:32

passed #214403
dind allowed to fail
docker_fedora-25-amd64

00:00:32

passed #214404
dind allowed to fail
docker_fedora-26-amd64

00:00:32

passed #214407
dind allowed to fail
docker_opensuse-42_3-amd64

00:00:31

passed #214408
dind allowed to fail
docker_ubuntu-14_04-amd64

00:00:31

passed #214409
dind allowed to fail
docker_ubuntu-16_04-amd64

00:00:33

 
  Build
passed #214416
docker linux amd64
build-centos-6-amd64

00:01:21

passed #214417
docker linux amd64
build-centos-7-amd64

00:01:21

passed #214410
docker linux amd64
build-debian-7-amd64

00:01:31

passed #214421
docker linux amd64
build-debian-7-i386

00:00:59

passed #214411
docker linux amd64
build-debian-8-amd64

00:01:22

passed #214422
docker linux amd64
build-debian-8-i386

00:01:03

passed #214412
docker linux amd64
build-debian-9-amd64

00:01:31

passed #214423
docker linux amd64
build-debian-9-i386

00:01:23

passed #214413
docker linux amd64
build-debian-testing-amd64

00:01:30

passed #214424
docker linux amd64
build-debian-testing-i386

00:01:04

passed #214414
docker linux amd64
build-fedora-25-amd64

00:01:22

passed #214415
docker linux amd64
build-fedora-26-amd64

00:01:22

passed #214425
freebsd amd64
build-freebsd-11-amd64

00:02:09

passed #214426
freebsd i386
build-freebsd-11-i386

00:02:03

passed #214418
docker linux amd64
build-opensuse-42_3-amd64

00:01:21

passed #214419
docker linux amd64
build-ubuntu-14_04-amd64

00:01:27

passed #214420
docker linux amd64
build-ubuntu-16_04-amd64

00:01:21