Extend "Links" section

[zeileis]

Thanks for all the improvements in the task view so far. When going through the task view, I noticed though that the links currently only list the SIG. I think that a few other links (such as rspatial.org, the geocomputation book, and other similar resources) would be worth listing at the end again. Would you consider extending the "Links" section in this direction?

[zeileis]

Thanks!