Document recommendations to ensure Black version compatibility

[akaihola]

• We can't guarantee that the current Darker version will work with any given future Black version (even the next one).
• There is no upper version limit for the Black dependency (because we don't want to prevent users from upgrading Black, and since we don't want to make a Darker release for each new Black release).
• Recommend pinning Black and only upgrading it explicitly after testing that it works with the version of Darker the user is running.

Originally posted by @akaihola in #334

[akaihola]

Superseded by #382.