Merge branch 'bug/250' into 'master'

Add a release note to 3.0.0 about --exclude

When we rewrote Flake8 for 3.0, we unintentionally stopped forcibly
excluding some patterns regardless of what the user specified. This note
is added after the fact to make it easier for users to determine the
cause of any backwards incompatibility they might notice.

Closes #250

See merge request !139