Summary of the discussion in #1499 (comment) and following posts:

For the given file structure

    └── examples
        ├── script_example.py
        ├── text_example.rst
        └── GALLERY_HEADER.rst

I've envisioned that it just picks up any .rst as it picks up any .py, without the need to explicitly mention them.

Basically, the conf

sphinx_gallery_conf = {
    ...
    'example_extensions': {'.py', '.rst'}
}

• The special files GALLERY_HEADER.rst / README.rst are always be excluded.
• This is an opt-in feature, i.e. 'example_extensions': {'.py',} stays as the default. and people have to add '.rst' to enable this.

Note: I'm currently working on this and have some small perparatory refactoring PRs (#1508, #1509, #1510) - just to let you know in which context they have been created.