add Transaction isolation docs (#434)

Author: aminalaee

docs/connections_and_transactions.md

@@ -98,4 +98,11 @@ async def create_users(request):
 Transaction blocks are managed as task-local state. Nested transactions
 are fully supported, and are implemented using database savepoints.
 
+Transaction isolation-level can be specified if the driver backend supports that:
+
+```python
+async with database.transaction(isolation="serializable"):
+    ...
+```
+
 [starlette]: https://github.com/encode/starlette

docs/index.md

@@ -17,8 +17,6 @@ expression language, and provides support for PostgreSQL, MySQL, and SQLite.
 Databases is suitable for integrating against any async Web framework, such as [Starlette][starlette],
 [Sanic][sanic], [Responder][responder], [Quart][quart], [aiohttp][aiohttp], [Tornado][tornado], or [FastAPI][fastapi].
 
-**Community**: [https://discuss.encode.io/c/databases](https://discuss.encode.io/c/databases)
-
 **Requirements**: Python 3.6+
 
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