Document where Gitaly instance writes logs during rspec

doc/development/gitaly.md

@@ -63,6 +63,8 @@ If your test-suite is failing with Gitaly issues, as a first step, try running:
 rm -rf tmp/tests/gitaly
 ```
 
+During rspec tests, the Gitaly instance will write logs to `gitlab/log/gitaly-test.log`.
+
 ## Legacy Rugged code
 
 While Gitaly can handle all Git access, many of GitLab customers still