Add developer guide (#638)

* doc: add skeleton of dev guide

Signed-off-by: Samuel <sam.r.c@hotmail.com>

* doc: do not specify patch version of dotnet

Signed-off-by: Samuel <sam.r.c@hotmail.com>

* doc: remove deleted sections

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* doc: add link to dev guide from readme
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* doc: apply code review comments

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* doc: remove excess lines
Signed-off-by: Samuel <sam.r.c@hotmail.com>

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Signed-off-by: Samuel <sam.r.c@hotmail.com>

DEVELOPER_GUIDE.md

@@ -0,0 +1,40 @@
+- [Developer Guide](#developer-guide)
+  - [Prerequisites](#prerequisites)
+  - [Install Docker Image](#install-docker-image)
+  - [Running Tests](#running-tests)
+  - [Client Code Generator](#client-code-generator)
+
+# Developer Guide
+
+## Prerequisites
+
+The .NET 6.0 SDK is required:
+
+```
+$ dotnet --list-sdks
+6.0.xxx
+```
+
+## Running Tests
+
+The integration tests will download opensearch and run a local cluster for you. To run the full suite of tests, all you need to do is call the below script.
+
+```
+.\build.bat
+```
+
+```
+build.sh
+```
+
+## Client Code Generator
+
+OpenSearch publishes an [OpenAPI specification](https://github.com/opensearch-project/opensearch-api-specification/releases/download/main/opensearch-openapi.yaml) in the [opensearch-api-specification](https://github.com/opensearch-project/opensearch-api-specification) repository, which is used to auto-generate the less interesting parts of the client.
+
+```
+./build.sh codegen --branch main
+```
+or
+```
+./build.bat codegen --branch main
+```

README.md

@@ -13,6 +13,7 @@
   - [OpenSearch.Client](#opensearch-client)
   - [Getting Started](#getting-started)
   - [Compatibility with OpenSearch](#compatibility-with-opensearch)
+  - [Developer Guide](#developer-guide)
   - [Code of Conduct](#code-of-conduct)
   - [License](#license)
   - [Copyright](#copyright)
@@ -55,6 +56,10 @@ See [USER_GUIDE](USER_GUIDE.md) to get started with the .NET client.
 
 See [COMPATIBILITY](COMPATIBILITY.md).
 
+## Developer Guide
+
+See [DEVELOPER_GUIDE](DEVELOPER_GUIDE.md).
+
 ## Code of Conduct
 
 This project has adopted the [Amazon Open Source Code of Conduct](CODE_OF_CONDUCT.md). For more information see the [Code of Conduct FAQ](https://aws.github.io/code-of-conduct-faq), or contact [opensource-codeofconduct@amazon.com](mailto:opensource-codeofconduct@amazon.com) with any additional questions or comments.