add hammerdb jobs readme

src/test/hammerdb/README.md

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+# How to trigger hammerdb benchmark jobs
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+You can trigger two types of hammerdb benchmark jobs:
+-ch_benchmark (analytical and transactional queries)
+-tpcc_bnehcmark (only transactional queries)
+
+If need to prepend `ch_benchmark/` or `tpcc_benchmark/` prefix to your branch to trigger these jobs
+relatively.
+
+You will see the results in [https://github.com/citusdata/release-test-results](https://github.com/citusdata/release-test-results).
+
+**You will need to delete the resource groups manually from portal, the resource groups are `citusbot_ch_benchmark_rg` and `citusbot_tpcc_benchmark_rg`.**
+
+These jobs use the hammerdb tool under the hood, please see [https://github.com/citusdata/test-automation#running-automated-hammerdb-benchmark](https://github.com/citusdata/test-automation#running-automated-hammerdb-benchmark) for more details.