Update FTShoe driver documentation indicating the output values reference body

[yeshasvitirupachuri]

Following our investigation on the multiplication by -1 in FTShoe driver we will update the documentation on the driver to clearly indicate why the design choice of -1 is made and on which body the FTShoes output values are acting.

[traversaro]

Relevant Gazebo tutorial and past discussion on this:

• http://gazebosim.org/tutorials?tut=force_torque_sensor&cat=sensors
• https://bitbucket.org/osrf/gazebo/issues/940/

[fjandrad]

@Yeshasvitvs did you do this at some point?

[yeshasvitirupachuri]

@fjandrad I did not do it!
Which documentation is apt to put in this information ?

[fjandrad]

Not sure, it might make sense to put it in the dedicated repo of the ftSkshoes even if the driver is here, since is related to reference frames more than the driver itself . @Yeshasvitvs