Documentation for Lifespan tasks

docs/lifespan_tasks.md

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+# Lifaspan tasks in PyNest
+
+## Introduction
+
+Lifespan tasks - coroutines, which run while app is working. 
+
+## Defining a lifespan task
+As example of lifespan task will use coroutine, which print time every hour. In real user cases can be everything else.
+
+```python
+import asyncio
+from datetime import datetime
+
+async def print_current_time():
+    while True:
+        current_time = datetime.now().strftime("%H:%M:%S")
+        print(f"Current time: {current_time}")
+        await asyncio.sleep(3600)
+```
+
+## Implement a lifespan task
+In `app_module.py` we can define a startup handler, and run lifespan inside it
+
+```python
+from nest.core import PyNestFactory
+
+app = PyNestFactory.create(
+    AppModule,
+    description="This is my PyNest app with lifespan task",
+    title="My App",
+    version="1.0.0",
+    debug=True,
+)
+
+http_server = app.get_server()
+
+@http_server.on_event("startup")
+async def startup():
+    await print_current_time()
+```
+
+Now `print_current_time` will work in lifespan after startup.