resistor-color: Add explanation for disabled tests

[senekor]

Issue that lead to the tests being disabled: https://github.com/exercism/exercism/issues/6353

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Hi there!

I've been doing a bit of mentoring for Rust lately and I've had three people ask about why the tests for resistor-color aren't running online. I suspect many others are confused, but don't ask via mentoring.

I thought it might be a good idea to include a note in the description.