Add section to developer guide on testing operations on images, etc.

[khughitt]

Often we will want to write a unit test for some function that operates on a large structure, such as an image.

A place has been provided to include test data (sunpy.data.test) for these kinds of operations, but so far now rules have been created about what should be included there.

It would therefore be helpful to decide on such rules and include a section in the developers guide on the topic.

Things to consider:

1. When should a new data file be added to sunpy.data.test?
2. What about hard-coding "fake data" (e.g. a 2-dimensional ndarray) instead of reading in a file each time? For example,
   we could include some sample 32x32 pixel image data, base64-encoded in sunpy.data.test.
3. Limits on filesize?
4. Optimal dimensions to ensure accurate testing? (16x16? 32x32?)

It's important to work this out early on so that developers can write good test code for data operations.

[khughitt]

First step: discuss rules in meeting or on mailing list.