mark release date

CHANGELOG.md

@@ -1,6 +1,7 @@
-# nanoflann 1.5.3: UNRELEASED
+# nanoflann 1.5.3: Released Dec 7, 2023
  * **Other changes**:
    - Save one redundant call to `computeMinMax()` in `middleSplit_` ([PR#220](https://github.com/jlblancoc/nanoflann/pull/220) by [qq422216549](https://github.com/qq422216549)).
+     This saves *a lot* of time, up to 20% faster in a benchmark with small (thousands) point clouds.
 
 # nanoflann 1.5.2: Released Nov 29, 2023
  * **Other changes**: