Added X-Plane to the list of software using VMA

The X-Plane Vulkan renderer is build around VMA as its memory allocation backend and has been shipping since late 2019. We make heavy use of the defragmentation feature and have very aggressive streaming of world data as well as texture paging. Planes are quite fast and earth is quite large, so we constantly move memory around and dynamically adjust things as the memory budget grows or shrinks and the user moves around. It's absolutely rock solid on top of VMA :)
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 # Software using this library
 
+- **[X-Plane](https://x-plane.com/)**
 - **[Detroit: Become Human](https://gpuopen.com/learn/porting-detroit-3/)**
 - **[Vulkan Samples](https://github.com/LunarG/VulkanSamples)** - official Khronos Vulkan samples. License: Apache-style.
 - **[Anvil](https://github.com/GPUOpen-LibrariesAndSDKs/Anvil)** - cross-platform framework for Vulkan. License: MIT.