Document release process.

Fixes #292.

This is mostly an initial sketch for discussion.

This also at least partially addresses #86, #87, #126, #277.
This fails when I try it.  After spending a long time downloading things:

```
Downloading ...6d9c01a9f829aa7333cbacff2a79-primary.sqlite.bz2:  7.57 KB/7.57 KB    
Copying gs://archive.clusterhq.com/fedora/20/x86_64/repodata/repomd.xml...
Downloading file:///tmp/tmpSSRJTb/x86_64/repomd.xml:             3.01 KB/3.01 KB    
Repository flocker is listed more than once in the configuration
http://build.clusterhq.com/results/fedora/20/x86_64/%25s/results/fedora/20/x86_64/0.1.0/repodata/repomd.xml: [Errno 14] HTTP Error 404 - Not Found
Trying other mirror.
flocker/primary_db             FAILED                                          
http://build.clusterhq.com/results/fedora/20/x86_64/%25s/results/fedora/20/x86_64/0.1.0/repodata/713bc74e5daa9586593a04538e2d0a87d0c0c389105e06332a03fd
7252778259-primary.sqlite.bz2: [Errno 14] HTTP Error 404 - Not Found
Trying other mirror.
http://build.clusterhq.com/results/fedora/20/x86_64/%25s/results/fedora/20/x86_64/0.1.0/repodata/713bc74e5daa9586593a04538e2d0a87d0c0c389105e06332a03fd
7252778259-primary.sqlite.bz2: [Errno 14] HTTP Error 404 - Not Found
Trying other mirror.
failure: repodata/713bc74e5daa9586593a04538e2d0a87d0c0c389105e06332a03fd7252778259-primary.sqlite.bz2 from flocker: [Errno 256] No more mirrors to try.
http://build.clusterhq.com/results/fedora/20/x86_64/%s/results/fedora/20/x86_64/0.1.0/repodata/713bc74e5daa9586593a04538e2d0a87d0c0c389105e06332a03fd72
52778259-primary.sqlite.bz2: [Errno 14] HTTP Error 404 - Not Found
Traceback (most recent call last):
  File "admin/upload-rpms", line 118, in <module>
    main(options)
  File "admin/upload-rpms", line 104, in main
    build_server=options['build-server'])
  File "admin/upload-rpms", line 89, in upload_rpms
    packages=FLOCKER_PACKAGES)
  File "admin/upload-rpms", line 66, in update_repo
    check_call([b'yumdownloader', b'-c', yum_repo_config.path, b'--disablerepo=*', b'--enablerepo=flocker', b'--destdir', rpm_directory.path] + package
s)
  File "/usr/lib64/python2.7/subprocess.py", line 542, in check_call
    raise CalledProcessError(retcode, cmd)
subprocess.CalledProcessError: Command '['yumdownloader', '-c', '/tmp/tmpSSRJTb/x86_64/build.repo', '--disablerepo=*', '--enablerepo=flocker', '--destd
ir', '/tmp/tmpSSRJTb/x86_64', 'python-flocker', 'flocker-cli', 'flocker-node']' returned non-zero exit status 1
```

I'm investigating but I don't really have a clue what's going on here.