Added example for client-side certificates validation

I've spent some time figuring this out on my own (and have written an answer to the only other guy on SO that asked himself the same question http://stackoverflow.com/questions/41701791/aiohttp-and-client-side-ssl-certificates/43664652#43664652)

Requests documentation is pretty straightforward about this http://docs.python-requests.org/en/master/user/advanced/#client-side-certificates

I think this will be a useful example for future users of this feature (without any high-level API changes like requests implemented).

docs/client.rst

@@ -505,6 +505,17 @@ pass it into the connector::
   session = aiohttp.ClientSession(connector=conn)
   r = await session.get('https://example.com')
 
+If you need to verify **client-side** certificates, you can do the same thing as the previous example,
+but add another call to ``load_cret_chain`` with the key pair::
+
+  sslcontext = ssl.create_default_context(
+     cafile='/path/to/client-side-ca-bundle.crt')
+  sslcontext.load_cert_chain('/path/to/client/public/key.pem', '/path/to/client/private/key.pem')
+  conn = aiohttp.TCPConnector(ssl_context=sslcontext)
+  session = aiohttp.ClientSession(connector=conn)
+  r = await session.get('https://server-with-client-side-certificates-validaction.com')
+
+
 You may also verify certificates via MD5, SHA1, or SHA256 fingerprint::
 
   # Attempt to connect to https://www.python.org