Advertise AnyReader in documentation

docs/topics/highlevel.rst

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+Highlevel APIs
+==============
+The :py:mod:`rosbags.highlevel` package provides classes that abstract the complexity of ROS types, serialization and message access into single easy-to-use interfaces.
+
+All in one reader
+-----------------
+Instances of the :py:class:`AnyReader <rosbags.highlevel.AnyReader>` class give unified access to ROS1 and ROS2 bag files. If a bag file includes message definitions the reader auto-registers all messages into a blank type store, otherwise it falls back to the default type store. It also exposes appropriate deserialization methods on the reader instance itself. 
+
+.. code-block:: python
+
+   from pathlib import Path
+
+   from rosbags.highlevel import AnyReader
+
+   # create reader instance and open for reading
+   with AnyReader([Path('/home/ros/rosbag_2020_03_24')]) as reader:
+       connections = [x for x in reader.connections if x.topic == '/imu_raw/Imu']
+       for connection, timestamp, rawdata in reader.messages(connections=connections):
+            msg = reader.deserialize(rawdata, connection.msgtype)
+            print(msg.header.frame_id)
+
+AnyReader takes a list of ``pathlib.Path`` instances as arguments. It can take either one ROS2 bag file or one or more ROS1 bag files belonging to a split bag. The reader will replay ROS1 split bags in correct timestamp order.