Add Squish hooks to Boot to Qt documentation

Now that we have Squish hooks in Boot to Qt we should add a few notes to Boot to Qt documentation:
-link to Squish documentation
-note that there are two main ways to start Squish hooks in the target device:
--environment variable
--starting squish server using a command line switch when starting the app on target

Examples:
in target device add a line to e.g.:
/etc/default/qt
QT_QPA_GENERIC_PLUGINS=squish:10100

or start app like this for instance:
/usr/bin/appcontroller /opt/examplename/bin/examplename -plugin squish:10100

When deploying from creator the env variable might be easier.

In the dev host follow the usual Squish IDE process, where you define the target device ip and port (same port as in target):
Edit -> Server Settings... -> Manage AUTs -> Attachable AUTs -> Add