Added the CONTRIBUTING guide with the @generated info

Author: ezimuel

CONTRIBUTING.md

@@ -0,0 +1,94 @@
+# Contributing to the PHP Elasticsearch Client
+
+If you have a bugfix or new feature that you would like to contribute to
+elasticsearch-php, please find or open an issue about it first. Talk about what
+you would like to do. It may be that somebody is already working on it, or that
+there are particular issues that you should know about before implementing the
+change.
+
+We enjoy working with contributors to get their code accepted. There are many
+approaches to fixing a problem and it is important to find the best approach
+before writing too much code.
+
+## Running Elasticsearch locally
+
+We've provided a script to start an Elasticsearch cluster of a certain version
+found at `.buildkite/run-elasticsearch.sh`.
+
+There are several environment variables that control integration tests:
+
+- `PYTHON_VERSION`: Version of Python to use, defaults to `3.9`
+- `TEST_SUITE`: Connection class to use, defaults to `Urllib3HttpConnection`
+- `STACK_VERSION`: Version of Elasticsearch to use. These should be
+  the same as tags of `docker.elastic.co/elasticsearch/elasticsearch`
+  such as `8.0.0-SNAPSHOT`, `7.x-SNAPSHOT`, etc. Defaults to the
+  same `*-SNAPSHOT` version as the branch.
+
+**NOTE: You don't need to run the live integration tests for all changes. If
+you don't have Elasticsearch running locally the integration tests will be skipped.**
+
+## API Code Generation
+
+All API methods in `src/Endpoints` and in `src/Traits/ClientEndpointsTrait.php` are
+automatically generated from the [Elasticsearch specification](https://github.com/elastic/elasticsearch-specification)
+and [rest-api-spec](https://github.com/elastic/elasticsearch/tree/master/rest-api-spec/src/main/resources/rest-api-spec/api).
+
+You can check if a PHP file has been generated searching for `@generated` tag
+in the source code (e.g. [here](https://github.com/elastic/elasticsearch-php/blob/main/src/Traits/ClientEndpointsTrait.php#L27)
+in the `ClientEndpointsTrait.php`).
+
+Any changes to these files should be avoid, you can submit to the Elasticsearch 
+specification project and will be imported the next time the client will be generated.
+The generator itself is currently a private project.
+
+## Contributing Code Changes
+
+The process for contributing to any of the Elasticsearch repositories is similar.
+
+1. Please make sure you have signed the [Contributor License
+   Agreement](http://www.elastic.co/contributor-agreement/). We are not
+   asking you to assign copyright to us, but to give us the right to distribute
+   your code without restriction. We ask this of all contributors in order to
+   assure our users of the origin and continuing existence of the code. You only
+   need to sign the CLA once.
+
+2. Run the linter and test suite to ensure your changes do not break existing code.
+   Run the last optional step only if you want to test your changes with the
+   integration tests. You need to specify the `STACK_VERSION` of Elasticsearch (e.g.
+   `8.17.0`), you can check the Elasticsearch versions [here](https://github.com/elastic/elasticsearch/releases).
+
+   ```
+   # Run PHPStan, see https://phpstan.org/
+   $ composer run-script phpstan
+   
+   # Run the unit tests
+   $ composer run-script test
+   
+   # Run the integration tests (optional)
+   $ STACK_VERSION="8.17.0" .buildkite/run-tests
+   ```
+
+
+3. Rebase your changes.
+   Update your local repository with the most recent code from the main
+   elasticsearch-php repository, and rebase your branch on top of the latest branch.
+   If you want to propose a change in the latest version, you need to use the `main`
+   branch. If you are proposing for `8.x` version you should use the latest
+   minor branch (e.g. `8.17`). If want to propose a change for the oldest versions
+   you need to use the `7.17` or `6.8.x` branches. Remember, we support only the latest
+   minor of the previous majour. For instance, if the latest version is `8.x` we
+   support the last minor of `7.x` (i.e. `7.17`).
+   We prefer your changes to be squashed into a single commit for easier
+   backporting.
+
+4. Submit a pull request. Push your local changes to your forked copy of the
+   repository and submit a pull request. In the pull request, describe what your
+   changes do and mention the number of the issue where discussion has taken
+   place, eg “Closes #123″.  Please consider adding or modifying tests related to
+   your changes.
+
+Then sit back and wait. There will probably be a discussion about the pull
+request and, if any changes are needed, we would love to work with you to get
+your pull request merged into elasticsearch-php.
+
+Thanks in advance for all your future contributions!

README.md

@@ -180,19 +180,8 @@ You can checkout the
 
 ## Contribute 🚀
 
-We welcome contributors to the project. Before you begin, some useful info...
-
-- If you want to contribute to this project you need to subscribe to a
-  [Contributor Agreement](https://www.elastic.co/contributor-agreement).
-- Before opening a pull request, please create an issue to
-  [discuss the scope of your proposal](https://github.com/elastic/elasticsearch-php/issues).
-- If you want to send a PR for version `8.0` please use the `8.0` branch, for
-  `8.1` use the `8.1` branch and so on.
-- Never send PR to `master` unless you want to contribute to the development
-  version of the client (`master` represents the next major version).
-- Each PR should include a **unit test** using [PHPUnit](https://phpunit.de/).
-  If you are not familiar with PHPUnit you can have a look at the
-  [reference](https://phpunit.readthedocs.io/en/9.5/).
+We welcome contributors to the project. You can refer to the [CONTRIBUTING](CONTRIBUTING.md)
+guide for more information.
 
 Thanks in advance for your contribution! :heart: