Add datamash.

Fixes #412.

README.md

@@ -221,6 +221,8 @@ Notes:
 
 - Know about `tee` to copy from stdin to a file and also to stdout, as in `ls -al | tee file.txt`.
 
+- For more complex calculations, including grouping, reversing fields, and statistical calculations, consider [`datamash`](https://www.gnu.org/software/datamash/).
+
 - Know that locale affects a lot of command line tools in subtle ways, including sorting order (collation) and performance. Most Linux installations will set `LANG` or other locale variables to a local setting like US English. But be aware sorting will change if you change locale. And know i18n routines can make sort or other commands run *many times* slower. In some situations (such as the set operations or uniqueness operations below) you can safely ignore slow i18n routines entirely and use traditional byte-based sort order, using `export LC_ALL=C`.
 
 - You can set a specific command's environment by prefixing its invocation with the environment variable settings, as in `TZ=Pacific/Fiji date`.