Move contributing guide to wiki

This avoids a "possibly invalid URL" NOTE from R CMD check on CRAN.

Author: joshuaulrich

.github/CONTRIBUTING.md

@@ -93,7 +93,7 @@ mailing list (you must subscribe to post).
 
 ### Contributing
 
-Please see the [contributing guide](.github/CONTRIBUTING.md).
+Please see the [Contributing Guide](https://github.com/joshuaulrich/TTR/wiki/Contributing-Guide).
 
 ### See Also