Update readme with dependencies for building

Closes #915
Add instructions for creating and using the docker group
remove extraneous step and stupid whitespace
registry: removing opaqueRequest
Merge pull request #1473 from shin-/978-opaque-v2

Reworking opaque requests in registry module
Merge pull request #1569 from TylerBrock/patch-1

Add instructions for creating and using the docker group
testing, issue #1431: Temporarely disable TestContainerTop from the testsuite
Merge pull request #1572 from dotcloud/1431-disable-TestContainerTop

Tests: testing, issue #1431: Temporarely disable TestContainerTop from the testsuite
Merge pull request #1550 from dotcloud/update-dependencies-in-readme

Update readme with dependencies for building
Show tag used when image is missing
Merge pull request #1575 from evanphx/tags

Show tag used when image is missing
Add an option to set the working directory.

This makes it possible to simply wrap a command inside a container. This makes
it easier to use a container as an unified build environment.

Examples:

~/workspace/docker
$ docker  run  -v `pwd`:`pwd` -w `pwd` -i -t  ubuntu ls
AUTHORS		 Makefile	archive.go	   changes.go	      docker
[...]


docker  run  -v `pwd`:`pwd` -w `pwd` -i -t  ubuntu pwd
/home/marco/workspace/docker
Merge pull request #1459 from mhennings/set-working-directory

Add an option to set the working directory