add custom_example section and exercise

[braingram]

I'll open this PR for review but at the moment I don't think encouraging custom_schema use is a good thing. It doesn't behave as I would expect. Here are a few issues (that will become asdf issues):

• providing a custom_schema on AsdfFile.__init__ doesn't cause a validation error when write_to is called: custom_schema not used on write_to asdf#1860
• asdf caches schemas and appears to cache based on file name (or something), this can lead to unexpected behavior if a schema file is change within an interactive session Changing custom_schema file results in unexpected validation asdf#1861