Feature request: include a descriptive paragraph when listing the behaviour of a Justfile

One of the attractions of `just` is getting a quick summary of the recipes with `--list`. I would find this more useful if it included a brief description of the project. The way I achieve this at the moment is to add the following recipe as the first in the file, which simply lists all the comment lines at the start of the file before listing the recipes.

```Just
# Summarises the purpose of this file.
# This lists the comment lines of the file until the first line that does
# not start with a '#' character. Then it lists the targets of the file.
about:
@awk '/^#/ {print} !/^#/ {exit}' "{{justfile()}}"
@echo
@just --list
```

This might be accommodated with
- a new option (e.g. `just --about`) or
- by a setting that sets a default for `--list-heading` (e.g. set list-heading "My very awesome project")
- by having a more elaborate convention for marking out the project summary (e.g. doubled '#' marks) and extending `--list`.