Add BENCHMARKING.md for sum.

src/uu/sum/BENCHMARKING.md

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+## Benchmarking `sum`
+
+<!-- spell-checker:ignore wikidatawiki -->
+
+Large sample files can for example be found in the [Wikipedia database dumps](https://dumps.wikimedia.org/wikidatawiki/latest/), usually sized at multiple gigabytes and comprising more than 100M lines.
+
+After you have obtained and uncompressed such a file, you need to build `sum` in release mode
+
+```shell
+$ cargo build --release --package uu_sum
+```
+
+and then you can time how it long it takes to checksum the file by running
+
+```shell
+$ /usr/bin/time ./target/release/sum wikidatawiki-20211001-pages-logging.xml
+```
+
+For more systematic measurements that include warm-ups, repetitions and comparisons, [Hyperfine](https://github.com/sharkdp/hyperfine) can be helpful. For example, to compare this implementation to the one provided by your distribution run
+
+```shell
+$ hyperfine "./target/release/sum wikidatawiki-20211001-pages-logging.xml" "/usr/bin/sum wikidatawiki-20211001-pages-logging.xml"
+```