Add changelog and comment.

CHANGELOG.md

@@ -1,5 +1,11 @@
 # Changelog
 
+## 0.12.3
+
+### Changes
+
+* Added `Sync` and `Send` implementations to `Snapshot` (pavel-mukhanov)
+
 ## 0.12.2 (2019-05-03)
 
 ### Changes

src/db.rs

@@ -104,6 +104,8 @@ pub struct Snapshot<'a> {
     inner: *const ffi::rocksdb_snapshot_t,
 }
 
+/// `Send` and `Sync` implementations for `Snapshot` are safe, because `Snapshot` is
+/// immutable and can be safely shared between threads.
 unsafe impl<'a> Send for Snapshot<'a> {}
 unsafe impl<'a> Sync for Snapshot<'a> {}