CONTRIBUTING: document the 'sam' tool

[dolmen]

Scripts in the doc directory use a sam tool which is not in the usual developer toolbox.

CONTRIBUTING.md or the wiki page should mention the source of that tool.

[dolmen]

https://en.wikipedia.org/wiki/Sam_(text_editor)
http://sam.cat-v.org/
Source code for Unix: https://github.com/9fans/plan9port/tree/master/src/cmd/sam
Manual: https://9fans.github.io/plan9port/man/man1/sam.html

[dolmen]

I've not yet found an easy path to install of sam on MacOS.

A container image with sam would be really helpful to contribute to the content of doc directory, which is the upstream source for Go's regexp/syntax documentation.