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 would be another good check:
```
$ python setup.py build
running build
running build_py
eduroam-3-163:asdf deil$ python setup.py build_sphinx
running build_sphinx
creating /Users/deil/temp/asdf/docs/_build
creating /Users/deil/temp/asdf/docs/_build/doctrees
creating /Users/deil/temp/asdf/docs/_build/html
creating /Users/deil/temp/asdf/docs/_static
running build
running build_py
Running Sphinx v1.2.3
Traceback (most recent call last):
  File "<stdin>", line 27, in <module>
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/site-packages/sphinx/application.py", line 107, in __init__
    confoverrides or {}, self.tags)
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/site-packages/sphinx/config.py", line 229, in __init__
    execfile_(filename, config)
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/site-packages/sphinx/util/pycompat.py", line 105, in execfile_
    exec(code, _globals)
  File "conf.py", line 80, in <module>
    __import__(setup_cfg['package_name'])
  File "/Users/deil/temp/asdf/build/lib.macosx-10.10-x86_64-3.4/asdf/__init__.py", line 15, in <module>
    from example_mod import *
ImportError: No module named 'example_mod'
Sphinx Documentation subprocess failed with return code 1
```
I'm not sure how to best describe what to do for this guide:
* Copy over `example_mod` to keep the instructions short and have something working very quickly?
* Tell the user to implement their first module and then continue with the instructions here?