add: contibuting.md

[ci skip]

Author: harisarang

CONTRIBUTING.md

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+# Typesense Java Client ☕ 🔍
+
+Java client for the Typesense API: https://github.com/typesense/typesense
+
+## Pull Request Process
+
+Please note we have a code of conduct, please follow it in all your interactions with the project.
+
+Before making a PR to this repository look out for the following guidelines.
+
+## Guidelines for updating the client
+
+- Head over to [Swagger Codegen](https://github.com/swagger-api/swagger-codegen) and download the cli jar.
+- Download the latest api spec for typesense server from [typesense-api-spec](https://github.com/typesense/typesense-api-spec)
+- Create a new `config.json` and add the following content
+  ```json
+  {
+    "modelPackage" : "org.typesense.model",
+    "apiPackage" : "org.typesense.api"
+  }
+  ```
+- Run the following command:
+  ```bash
+  java -jar swagger-codegen-cli-3.0.20.jar generate -i <path-to-spec> -l jaxrs-cxf-client -c <path-to-config.json> -o <out-dir>
+  ```
+- Now, copy the content under ```<out-dir>/src/gen/java/org/typesense/model``` and replace it with the content of the ```typesense-java/src/main/java/org/typesense/model``` folder in the `typesense-java` client repository.
+- And then make the necessary changes in `api` folder.
+
+**NOTE**: The `model` directory is **read-only** make sure not to edit it.
+