Add instructions for pruning the benchmark history (#1077)

prune-benchmark-history.md

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+# Pruning the benchmarks report page
+
+Benchmarks are run on every commit, even in pull requests. This causes the historic graphs on
+https://zio.github.io/zio-kafka/dev/bench/ to contain so many results that it becomes unreadable. Rather than limiting
+the number of items in the graph (which is supported by the github action that generates the page), we keep the
+benchmark results of _all_ master commits, but only keep 1 month of pull request commits.
+
+The process is currently manual. Here are the instructions:
+
+* Make sure `jq` is installed
+* Checkout the `gh-pages` branch
+* Execute the following command in the root of the project:
+  ```shell
+  scripts/prune-benchmark-history.sh
+  ```
+* Commit and push changes