[SYCL][UR][Doc] Add README.md for the Unified Runtime directory (#9939)

Add a readme to point contributors to the UR repository so PI changes
which require changes to the UR spec are correctly handled

Author: callumfare

sycl/plugins/unified_runtime/README.md

@@ -0,0 +1,22 @@
+# Unified Runtime
+
+This directory contains the implementation of the PI plugin for Unified Runtime,
+including the pi2ur translation layer, as well as the sources for the individual
+Unified Runtime adapters.
+
+## Making changes to Unified Runtime
+If you introduce changes to PI (e.g. new entry points, new enum values) you
+should introduce matching changes to the Unified Runtime spec.
+
+To do this, open a Pull Request adding the changes to the
+[Unified Runtime](https://github.com/oneapi-src/unified-runtime)
+repository, making sure to follow the
+[Contribution Guide](https://oneapi-src.github.io/unified-runtime/core/CONTRIB.html).
+
+When your changes to Unified Runtime are merged, you should:
+* Update the UR commit used by changing the `UNIFIED_RUNTIME_TAG` value in
+  [`CMakeLists.txt`](CMakeLists.txt)
+* Make changes to [`pi2ur.hpp`](pi2ur.hpp) to ensure correct mapping from PI to
+  UR
+* Make changes to the affected adapter implementations in the
+[`ur/adapters`](ur/adapters) folder