link performance testing wiki page from CONTRIBUTING.md (#7016)

* link performance testing wiki

* move entry around

---------

Co-authored-by: Michael Chirico <[email protected]>

Author: tdhock

.github/CONTRIBUTING.md

@@ -76,6 +76,10 @@ See [`?test`](https://rdatatable.gitlab.io/data.table/reference/test.html).
 1. **[Squashing Github pull requests into a single commit](http://eli.thegreenplace.net/2014/02/19/squashing-github-pull-requests-into-a-single-commit)**.
 1. **[Github help](https://help.github.com/articles/using-pull-requests/)** - you'll need the *fork and pull* model.
 
+#### Performance testing
+
+If your PR may have an effect on time/memory usage, please consider adding a performance test, either in the same PR, or a follow-up PR. See the [Performance testing](https://github.com/Rdatatable/data.table/wiki/Performance-testing) wiki page for details.
+
 Minimal first time PR
 ---------------------