Add explanatory header text to pymodbus.client

The subpackages TOC tree is large and messy, and the most useful sections (the
client common mixin and the synchronous client) are buried well below the fold,
with no TOC entries for them.

Author: camtarn

doc/source/library/pymodbus.client.rst

@@ -1,6 +1,9 @@
 pymodbus\.client package
 ========================
 
+Pymodbus offers a :mod:`synchronous client <pymodbus.client.sync>`, and async clients based on :mod:`asyncio <pymodbus.client.asynchronous.async_io>`, :mod:`Tornado <pymodbus.client.asynchronous.tornado>`, and :mod:`Twisted <pymodbus.client.asynchronous.Twisted>`.
+
+Each client shares a :mod:`common client mixin <pymodbus.client.common>` which offers simple methods for reading and writing.
 
 Subpackages
 -----------