adding mobility layer2 config created with ONDC rules

layer2/docs/generating_layer2.md

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+# Steps to generate a layer 2 config
+
+## Introduction
+There could be many ways to create a layer 2 config. This document introduces one possible way using which a layer 2 config can be created.
+
+## Details
+In this approach we are using OpenAPI 3.1 to construct a layer 2 config.
+
+[This](./../samples/mobility_1.1.0.yaml) Mobility domain layer 2 configuration was created using the above approach.
+
+The details of how to create a layer 2 configuration using this approach is documented [here](https://github.com/beckn/mobility/blob/layer2-oas-3.1/api/l2-config/oas-3.1/ondemandride/README.md).