Add documentation for Cached IK Kinematic Plugin

[BryceStevenWilley]

Was looking at https://github.com/ros-planning/pull/612, and there were some comments about adding that documentation to the tutorials. Should just be a simple copy-paste from the README file of the plugin.

I might some time to tackle this soon.

[welcome]

Thanks for reporting an issue. We will have a look asap. If you can think of a fix, please consider providing it as a pull request.

[felixvd]

@mamoll ?

[mamoll]

@felixvd thanks for the reminder! I just created a PR.