added hello-world test prompt

prompts/hello-world/README.md

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+# Hello World
+> A simple prompt as an example
+
+
+## Configuration
+- input_variables: []
+  - There are no inputs
+- output_parser: null
+  - There is no output parsing needed
+- template: 'Say hello world.'
+  - Just a simple hello.
+template_format: f-string
+  - We use standard f-string formatting here.
+
+## Usage
+
+Ex:
+```python3
+from langchain.prompts import load_from_hub
+from langchain.llms import OpenAI
+from langchain.prompts.loading import load_prompt
+
+llm = OpenAI(temperature=0.9)
+# prompt = load_from_hub("hello-world/prompt.yaml")
+output = llm(prompt.format())
+print(output)
+```

prompts/hello-world/prompt.yaml

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+input_variables: []
+output_parser: null
+template: 'Say hello world.'
+template_format: f-string