Update 5.x changelog (#513)

Docs/ChangeLog-5x.md

@@ -6,6 +6,25 @@ release of the 5.x series.
 All performance data on this page is measured on an Intel Core i5-9600K
 clocked at 4.2 GHz, running `astcenc` using AVX2 and 6 threads.
 
+
+<!-- ---------------------------------------------------------------------- -->
+## 5.1.0
+
+**Status:** In development.
+
+The 5.1.0 release is a maintenance release.
+
+* **General:**
+  * **Feature:** Added a new CMake build option to control use of native
+    gathers, as they can be slower than scalar loads on some common x86
+    microarchitectures. Build with `-DASTCENC_X86_GATHERS=OFF` to disable use
+    of native gathers in AVX2 builds.
+  * **Optimization:** Added new `gather()` abstraction for gathers using byte
+    indices, allowing implementations without gather hardware to skip the
+    byte-to-int index conversion.
+  * **Optimization:** Added improved intrinsics sequence for SSE and AVX2
+    `hmin()` and `hmax()`.
+
 <!-- ---------------------------------------------------------------------- -->
 ## 5.0.0