chore: add Buffer breaking change to changelog

[cratelyn]

in #635, some subtle breaking changes were made to how Buffer works.

this is documented in the description of that PR, here:

I had to change some of the integration tests slightly as part of this
change. This is because the buffer implementation using semaphore
permits is very subtly different from one using a bounded channel. In
the Semaphore-based implementation, a semaphore permit is stored in
the Message struct sent over the channel. This is so that the capacity
is used as long as the message is in flight. However, when the worker
task is processing a message that's been recieved from the channel,
the permit is still not dropped. Essentially, the one message actively
held by the worker task also occupies one "slot" of capacity, so the
actual channel capacity is one less than the value passed to the
constructor, once the first request has been sent to the worker. The
bounded MPSC changed this behavior so that capacity is only occupied
while a request is actually in the channel, which broke some tests
that relied on the old (and technically wrong) behavior.

bear particular attention to this:

The bounded MPSC changed this behavior so that capacity is only
occupied while a request is actually in the channel, which broke some
tests that relied on the old (and technically wrong) behavior.

this is a change in behavior that might affect downstream callers.

this commit adds mention of these changes to the changelog, to help consumers navigate the upgrade from tower 0.4 to 0.5.