Fix README's and update getting started (#352)

dotnet/README.md

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+# Get Started with Microsoft Agent Framework for C# Developers
+
+## Run the Minimal Console demo
+
+The Minimal Console demo is a simple console application which shows how to create and run an agent.
+
+Supported Platforms:
+- .Net: net9.0, net8.0, netstandard2.0, net472 
+- OS: Windows, macOS, Linux
+
+If you want to use the latest published packages following the instructions [here](../docs/FAQS.md).
+
+### 1. Configure required environment variables
+
+This samples uses Azure OpenAI by default so you need to set the following environment variable
+
+``` powershell
+$env:AZURE_OPENAI_ENDPOINT = "https://<your deployment>.openai.azure.com/"
+```
+
+If you want to use OpenAI
+
+1. Edit [Program.cs](./demos/MinimalConsole/Program.cs) and change the following lines:
+    ```csharp
+    AIAgent agent = new AzureOpenAIClient(
+      new Uri(Environment.GetEnvironmentVariable("AZURE_OPENAI_ENDPOINT")!),
+      new AzureCliCredential())
+       .GetChatClient("gpt-4o-mini")
+       .CreateAIAgent(
+         instructions: "You are a helpful assistant, you can help the user with weather information.",
+         tools: [AIFunctionFactory.Create(GetWeather)]);
+    ```
+    To this:
+    ```csharp
+    AIAgent agent = new OpenAIClient(Environment.GetEnvironmentVariable("OPENAI_API_KEY")!)
+      .GetChatClient("gpt-4o-mini")
+      .CreateAIAgent(
+        instructions: "You are a helpful assistant, you can help the user with weather information.",
+        tools: [AIFunctionFactory.Create(GetWeather)]);
+    ```
+2. Create an environment variable with your OpenAI key 
+    ``` powershell
+    $env:OPENAI_API_KEY = "sk-..."
+    ```
+
+### 2. Build the project
+
+```powershell
+cd demos\MinimalConsole
+dotnet build
+```
+
+### 3. Run the demonstration
+
+``` powershell
+dotnet run --framework net9.0 --no-build
+```
+
+Sample output:
+
+```
+The weather in Amsterdam is currently cloudy, with a high temperature of 15°C.
+```
+