[OpenXLA] Add openxla.md (#5507)

* Create openxla.md

docs/openxla.md

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+# OpenXLA
+
+As of June 28th, 2023, PyTorch/XLA now pulls XLA from OpenXLA. 
+OpenXLA is an [open source machine learning compiler XLA for GPUs, CPUs, and ML accelerators](https://github.com/openxla/xla). 
+
+Previous to OpenXLA, PyTorch/XLA pulled XLA directly from [TensorFlow](https://github.com/tensorflow/tensorflow/tree/master/tensorflow/compiler/xla). With our [XLA to OpenXLA migration](https://github.com/pytorch/xla/pull/5202), PyTorch/XLA now pulls XLA from [OpenXLA](https://github.com/openxla/xla).
+
+# How to use OpenXLA
+
+For [PJRT runtime](https://github.com/pytorch/xla/blob/master/docs/pjrt.md) users, there is no change with this migration. For XRT runtime users, there is a separate [XRT branch of PyTorch/XLA](https://github.com/pytorch/xla/tree/xrt) since OpenXLA doesn't support XRT.
+
+
+# Performance
+Below is a performance visual comparison of throughput for ResNet50 pre and post the migration on different TPU hardwares.
+
+| | resnet50-pjrt-v2-8 | resnet50-pjrt-v4-8 | resnet50-pjrt-v4-32 |
+| :------------  | :------------  | :------------  | :------------  |
+| Pre Migration  | 18.59    | 20.06 | 27.92 |
+| Post Migration | 18.63    | 19.94 | 27.14 |