document that the todo.txt priority system is intentionally unsupported

We intentionally do not support the todo.txt priority system — (A), (B) and friends aren't something i want to use.

Right now ParseTask still checks for a leading 3-character (...) token, but only so it knows where the */~ modifiers start. The priority token itself falls through into Description, so it renders as part of the task text and plays no part in sorting.

This issue is just to document that omission (README and/or a comment on the parser) so it doesn't read as an oversight, and to decide whether the tag should keep showing up in the description or be stripped. No behavior change intended beyond clarity.