Add python-searchlightclient to list of adopters.

This adds python-searchlightclient to the list of adopters.

Change-Id: Ifb6f1931c00fc735a0b0fcc830979a4d83c0527d

Author: ttripp

doc/source/plugins.rst

@@ -37,6 +37,7 @@ python-manilaclient            using argparse
 python-mistralclient           using OpenStackClient
 python-muranoclient            using argparse
 python-saharaclient            using OpenStackClient
+python-searchlightclient       using OpenStackClient
 python-troveclient             using argparse
 python-tuskarclient            using OpenStackClient
 python-zaqarclient             using OpenStackClient