Add comment to the new VmaVector constructor

See #74
diff --git a/src/vk_mem_alloc.h b/src/vk_mem_alloc.h
index a8e083e..623f860 100644
--- a/src/vk_mem_alloc.h
+++ b/src/vk_mem_alloc.h
@@ -4097,6 +4097,8 @@
     {

     }

     

+    // This version of the constructor is here for compatibility with pre-C++14 std::vector.

+    // value is unused.

     VmaVector(size_t count, const T& value, const AllocatorT& allocator)

         : VmaVector(count, allocator) {}