Document use of atomic wrapper
Document how to use `sponge` to atomic update textfiles.
These scripts are examples to be used with the Node Exporter Textfile
Collector.
+To use these scripts, we recommend using a `sponge` to atomically write the output.
+
+ <collector_script> | sponge <output_file>
+
+Sponge comes from [moreutils](https://joeyh.name/code/moreutils/)
+* [brew install moreutils](http://brewformulas.org/Moreutil)
+* [apt install moreutils](https://packages.debian.org/search?keywords=moreutils)
+
For more information see:
https://github.com/prometheus/node_exporter#textfile-collector
