docs: mention pytest-profiling

Could be useful for analysis of performance problems.

docs/contributing.rst

@@ -109,6 +109,11 b' To run these tests::'
 
     env TEST_PERFORMANCE=1 py.test kallithea/tests/performance
 
+To analyze performance, you could install pytest-profiling_, which enables the
+--profile and --profile-svg options to py.test.
+
+.. _pytest-profiling: https://github.com/manahl/pytest-plugins/tree/master/pytest-profiling
+
 
 Contribution guidelines
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