Small performance test documentation. (#5408)

* Docs for performance tests

* docfix

* Better formating

* More English

* Run example

* Update README.md

dbms/tests/performance/README.md

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+## ClickHouse performance tests
+
+This directory contains `.xml`-files with performance tests for `clickhouse-performance-test` tool.
+
+### How to write performance test
+
+First of all you should check existing tests don't cover your case. If there are no such tests than you should write your own.
+
+There two types of performance tests:
+* First is executed in loop, and have tag `<type>loop</type>` in config.
+* Second one is executed only once and have tag `<type>once</type>` in config.
+
+Type `once` should be used only for endless queries. Even if your query really long (10 seconds+), it's better to choose `loop` test.
+
+After you have choosen type, you have to specify `preconditions`. It contains table names. Only `hits_100m_single`, `hits_10m_single`, `test.hits` are available in CI.
+
+The most important part of test is `stop_conditions`. For `loop` test you should always use `min_time_not_changing_for_ms` stop condition. For `once` test you can choose between `average_speed_not_changing_for_ms` and `max_speed_not_changing_for_ms`, but first is preferable. Also you should always specify `total_time_ms` metric. Endless tests will be ignored by CI.
+
+`metrics` and `main_metric` settings are not important and can be ommited, because `loop` tests are always compared by `min_time` metric and `once` tests compared by `max_rows_per_second`.
+
+You can use `substitions`, `create`, `fill` and `drop` queries to prepare test. You can find examples in this folder.
+
+Take into account, that these tests will run in CI which consists of 56-cores and 512 RAM machines. Queries will be executed much faster than on local laptop.
+
+### How to run performance test
+
+You have to run clickhouse-server and after you can start testing:
+
+```
+$ clickhouse-performance-test --input-file my_lovely_test1.xml --input-file my_lovely_test2.xml
+$ clickhouse-performance-test --input-file /my_lovely_test_dir/
+```