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?
Here should be some verification if the package is set up OK.

I get this error:
```
eduroam-3-163:asdf deil$ python setup.py test --coverage
Freezing version number to asdf/version.py
running test
running build
running build_py
creating build
creating build/lib.macosx-10.10-x86_64-3.4
creating build/lib.macosx-10.10-x86_64-3.4/asdf
copying asdf/__init__.py -> build/lib.macosx-10.10-x86_64-3.4/asdf
copying asdf/_astropy_init.py -> build/lib.macosx-10.10-x86_64-3.4/asdf
copying asdf/conftest.py -> build/lib.macosx-10.10-x86_64-3.4/asdf
copying asdf/version.py -> build/lib.macosx-10.10-x86_64-3.4/asdf
Fixing build/lib.macosx-10.10-x86_64-3.4/asdf/__init__.py build/lib.macosx-10.10-x86_64-3.4/asdf/_astropy_init.py build/lib.macosx-10.10-x86_64-3.4/asdf/conftest.py build/lib.macosx-10.10-x86_64-3.4/asdf/version.py
Skipping implicit fixer: buffer
Skipping implicit fixer: idioms
Skipping implicit fixer: set_literal
Skipping implicit fixer: ws_comma
Fixing build/lib.macosx-10.10-x86_64-3.4/asdf/__init__.py build/lib.macosx-10.10-x86_64-3.4/asdf/_astropy_init.py build/lib.macosx-10.10-x86_64-3.4/asdf/conftest.py build/lib.macosx-10.10-x86_64-3.4/asdf/version.py
Skipping implicit fixer: buffer
Skipping implicit fixer: idioms
Skipping implicit fixer: set_literal
Skipping implicit fixer: ws_comma
error: [Errno 2] No such file or directory: '/var/folders/sb/4qv5j4m90pz1rw7m70rj1b1r0000gn/T/asdf-test-47jg1awa/lib.macosx-10.10-x86_64-3.4/asdf/tests/coveragerc'
eduroam-3-163:asdf deil$ find . -name 'coveragerc'
./asdf/tests/coveragerc
```

I think what's missing is to declar the coveragerc file as package data so that it's copied over to the build folder?