Link to the inline-r tests in the documentation.

[facundominguez]

The documentation does not show examples of calling Haskell functions from R, but there are some examples in the tests. Maybe the tests could be linked for this and other features while there are no better sources.

[facundominguez]

Another example of a feature implied but not shown in documentation is using mutable vectors in quasiquotations, which is rehearsed in test-qq.hs.