README: link to gh-pages Doxygen Documentation

I added our Doxygen documentation to *libSplash*'s [github pages](http://computationalradiationphysics.github.io/libSplash/).

Please note, that this branch is totally unrelated (an [orphan](https://help.github.com/articles/creating-project-pages-manually)) compared to our other branches.

To [update](https://github.com/ComputationalRadiationPhysics/libSplash/blob/gh-pages/update.sh) the documentation, run
```bash
git checkout gh-pages
./update.sh <branchName>
```
If you specify no name, `master` will be the default branch to create the documentation for.