Write instructions how to extend the library using the public API and where to find supporting libraries #642

Starting from release 3.0 library has methods in the public API which are meant used programmatically. It would be good idea to write instructions and examples how that public API can be used to extend the library.

There are many libraries which already extend the Selenium2Library in one way or other. It would be good to have links to point to other projects which go nicely together with SL. Like in #638

1. How to extend the library by providing PR with GitHub. This is mostly done in CONTRIBUTING.rst but should provide link to the right place.
2. How to extend the library with inheritance
3. How to get the library instance from Robot Framework
4. How to to extend the library without creating a new library to Robot Framework side.

Put the extending documentation in to the ./docs/extending/ folder.