Added instructions on setting up affiliated packages manually

@cdeil - as discussed previously... this is overdue!

@Cadair - you may also want to review this.

Preview: http://www.mpia-hd.mpg.de/~robitaille/astropy/docs_affiliated/development/affiliated-packages.html

This includes a section on updating for the manual method, but still needs instructions for updating when using the git method. I can try and add that later. In the mean time, a review of what's here already would be good.

**Note 1:** this may still contain mistakes because I haven't gone through and tried it out, but it would be helpful if someone could try it out just to see if they end up with a functional package.

**Note 2:** I changed ``yourpkg`` to ``packagename`` to match the package template otherwise it's confusing.

Question: is it worth splitting the two main sections into sub-pages now that it's all quite long?
This currently fails:
```
$ python setup.py test --coverage
Traceback (most recent call last):
  File "setup.py", line 18, in <module>
    from astropy_helpers.setup_helpers import (
  File "/private/tmp/kronkalonka/astropy_helpers/astropy_helpers/__init__.py", line 2, in <module>
    from .version import version as __version__
  File "/private/tmp/kronkalonka/astropy_helpers/astropy_helpers/version.py", line 151, in <module>
    version = update_git_devstr(_last_generated_version)
  File "/private/tmp/kronkalonka/astropy_helpers/astropy_helpers/version.py", line 34, in update_git_devstr
    devstr = get_git_devstr(sha=True, show_warning=False, path=path)
  File "/private/tmp/kronkalonka/astropy_helpers/astropy_helpers/version.py", line 144, in get_git_devstr
    return _decode_stdio(stdout)[:40]
  File "/private/tmp/kronkalonka/astropy_helpers/astropy_helpers/version.py", line 17, in _decode_stdio
    text = stream.decode(stdio_encoding)
AttributeError: 'str' object has no attribute 'decode'
```
Is this a known issue?