doc(common): Add the changelog file.

Author: Hywan

crates/matrix-sdk-common/changelog.d/6674.fixed.md

@@ -0,0 +1,10 @@
+Dropping `CrossProcessLock` no longer releases the lock if a `CrossProcessGuard`
+is still alive.
+
+An important problem has been found with the `CrossProcessLock`. We noticed
+that when a `CrossProcessLock` is dropped, all `CrossProcessLockGuard`s report
+0 holder, and thus, the lock is released, but it's wrong. As long as guards are
+alive, the lock **MUST NOT** be released. Dropping all the guards releases the
+lock, but dropping the `CrossProcessLock` —which is not the lock in itself— must
+not dropped the acquired lock.
+There is no public changes, but the incorrect behaviour is now fixed.