CONTRIBUTING.md:add link to TODO wiki page

[skip ci]

Author: andy5995

.github/CONTRIBUTING.md

@@ -42,6 +42,7 @@ contributing to the Slic3r project.
 	* [Help Wanted tasks](https://github.com/slic3r/Slic3r/labels/help%20wanted): pick one of them!
 	* [More available tasks](https://github.com/slic3r/Slic3r/milestone/32): let's discuss together before you start working on them
 	* Please comment in the related GitHub issue that you are working on it so that other people know.
+* Review the [TODO wiki page](https://github.com/slic3r/Slic3r/wiki/TODO).
 * Contribute to the [Manual](http://manual.slic3r.org/)! (see its [GitHub repository](https://github.com/slic3r/Slic3r-Manual))
 * You can also find us in #slic3r on [FreeNode](https://webchat.freenode.net): talk to _Sound_, _LoH_ or the other members of the Slic3r community.
 * Add an [issue](https://github.com/slic3r/Slic3r/issues) to the GitHub tracker if it isn't already present.