fix(py/samples/prompt_demo): add readme with instructions (#3983)

Author: yesudeep

py/samples/prompt_demo/README.md

@@ -0,0 +1,24 @@
+An example demonstrating how to manage prompts using Genkit's prompt loading system.
+
+## Setup environment
+
+Obtain an API key from [ai.dev](https://ai.dev).
+
+Export the API key as env variable `GEMINI_API_KEY` in your shell
+configuration.
+
+```bash
+export GEMINI_API_KEY='<Your api key>'
+```
+
+## Run the sample
+
+```bash
+genkit start -- uv run src/prompt_demo.py
+```
+
+## Prompt Structure
+
+- `data/`: Contains `.prompt` files (using [Dotprompt](https://genkit.dev/docs/dotprompt)).
+- `data/_shared_partial.prompt`: A partial that can be included in other prompts.
+- `data/nested/nested_hello.prompt`: A prompt demonstrating nested structure and partial inclusion.