Add layer readme

layer_gpu_support/README_LAYER.md

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+# Layer: GPU Support
+
+This layer is a tech support trick box that is designed to help diagnose causes
+of functional and performance issues in applications. It works by letting you
+quickly test your application with a set of API behavior overrides applied,
+which can help to identify likely problem areas in the application if an
+override causes an issue to disappear.
+
+## Using this layer
+
+This layer requires you to provide a configuration file that specifies the
+overrides to apply. Details of the configuration options in each override
+group are document in the Behavior overrides section below.
+
+The [`layer_config.json`](layer_config.json) file in this directory is a
+complete template configuration file you can start from. However, note that it
+does not enable any overrides by default, so running the layer with this
+configuration used "as is" will not do anything useful. Take a copy and
+modify it to enable the options you want to try.
+
+For Android you can run this layer with the following command line:
+
+```sh
+python3 lgl_android_install.py --layer layer_gpu_support --config <your.json>
+```
+
+## What devices are supported?
+
+This layer is device agnostic and should support any GPU.
+
+## Behavior overrides
+
+The current override groups are supported:
+
+* **Serialization:** control serialization of GPU workload scheduling to
+  diagnose issues caused by missing queue or command stream synchronization.
+
+### Serialization
+
+The serialization overrides allow forceful serialization of submitted
+workloads, ensuring that they run in queue submit order. The synchronization
+can be configured per workload type, allowing control over where serialization
+is added to command buffers and queues.
+
+#### Configuration options
+
+The configuration file allows control over specific options, as well as
+convenience options to force disable or enable all serialization.
+
+* If the `none` option is `true` then no serialization is applied, irrespective
+of other settings.
+* Else, if the `all` option is `true` then all serialization is applied,
+irrespective of other settings.
+* Else, the individual options are applied as specified.
+
+```jsonc
+"serialize": {
+    "none": false,          // Enable no serialization options
+    "all": false,           // Enable all serialization options
+    "queue": false,         // Enable cross-queue serialization of submits
+    "commandstream": {
+        "compute": {
+            "pre": false,   // Insert full barrier before dispatches
+            "post": false   // Insert full barrier after dispatches
+        },
+        "renderpass": {
+            "pre": false,   // Insert full barrier before render passes
+            "post": false   // Insert full barrier after render passes
+        },
+        "tracerays": {
+            "pre": false,   // Insert full barrier before trace rays
+            "post": false   // Insert full barrier after trace rays
+        },
+        "transfer": {
+            "pre": false,   // Insert full barrier before transfers
+            "post": false   // Insert full barrier after transfers
+        }
+    }
+}
+```
+
+- - -
+
+_Copyright © 2025, Arm Limited and contributors._