Update README.md

This example can be run faster for openai users.

README.md

@@ -31,8 +31,11 @@ pip install crewai
 2. **Setting Up Your Crew**:
 
 ```python
+import os
 from crewai import Agent, Task, Crew, Process
 
+os.environ["OPENAI_API_KEY"] = "Your Key"
+
 # Define your agents with roles and goals
 researcher = Agent(
   role='Researcher',