Add README for shared_pathways_service

Shared Pathways Service is a new feature to accelerate developer iteration by decoupling service creation for Pathways from code development.

Author: guptaaka

pathwaysutils/experimental/shared_pathways_service/README.md

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+# Shared Pathways Service
+
+The Shared Pathways Service accelerates developer iteration by providing a
+persistent, multi-tenant TPU environment. This decouples service creation from
+the development loop, allowing JAX clients to connect on-demand from a familiar
+local environment (like a laptop or cloud VM) to a long-running Pathways
+service that manages scheduling and error handling.
+
+## Requirements
+
+Make sure that your GKE cluster is running the Resource Manager and Worker pods.
+You can follow the steps
+<a href="https://docs.cloud.google.com/ai-hypercomputer/docs/workloads/pathways-on-cloud/troubleshooting-pathways#health_monitoring" target="_blank">here</a>
+to confirm the status of these pods. If you haven't started the Pathways pods
+yet, you can use [pw-service-example.yaml](yamls/pw-service-example.yaml).
+Make sure to modify the following values to deploy these pods:
+
+- A unique Jobset name for the cluster's Pathways pods
+- GCS bucket path
+- TPU type and topology
+- Number of slices
+
+These fields are highlighted in the YAML file with trailing comments for easier
+understanding.
+
+## Instructions
+
+1. Clone `pathwaysutils`.
+
+`git clone https://github.com/AI-Hypercomputer/pathways-utils.git`
+
+2. Install portpicker
+
+`pip install portpicker`
+
+3. Import `isc_pathways` and move your workload under
+`with isc_pathways.connect()` statement. Refer to
+[run_connect_example.py](run_connect_example.py) for reference. Example code:
+
+```
+ from pathwaysutils.experimental.shared_pathways_service import isc_pathways
+
+ with isc_pathways.connect(
+     cluster="my-cluster",
+     project="my-project",
+     region="region",
+     gcs_bucket="gs://user-bucket",
+     pathways_service="pathways-cluster-pathways-head-0-0.pathways-cluster:29001",
+     expected_tpu_instances={"tpuv6e:2x2": 2},
+ ) as tm:
+   import jax.numpy as jnp
+   import pathwaysutils
+   import pprint
+
+   pathwaysutils.initialize()
+   orig_matrix = jnp.zeros(5)
+   ...
+```
+
+The connect block will deploy a proxy pod dedicated to your client and connect
+your local runtime environment to the proxy pod via port-forwarding.