Add docstring

Author: timkpaine

fsspec/implementations/chained.py

@@ -6,4 +6,17 @@
 
 
 class ChainedFileSystem(AbstractFileSystem):
+    """Chained filesystem base class.
+
+    A chained filesystem is designed to be layered over another FS.
+    This is useful to implement things like caching.
+
+    This base class does very little on its own, but is used as a marker
+    that the class is designed for chaining.
+
+    Right now this is only used in `url_to_fs` to provide the path argument
+    (`fo`) to the chained filesystem from the underlying filesystem.
+
+    Additional functionality may be added in the future.
+    """
     protocol: ClassVar[str] = "chained"