Clarify about lack of atime testing on fork branches (#7018)

Author: MichaelChirico

.github/CONTRIBUTING.md

@@ -78,7 +78,7 @@ See [`?test`](https://rdatatable.gitlab.io/data.table/reference/test.html).
 
 #### 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.
+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. Note that first-time contributors _must_ do so in a follow-up PR, since the tests are only run on PRs from branches created directly in the Rdatatable/data.table repo. See the [Performance testing](https://github.com/Rdatatable/data.table/wiki/Performance-testing) wiki page for details.
 
 Minimal first time PR
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