Doc & Example for External Match Syntax

[mej]

NHC's support for Match Strings is a powerful, extensible mechanism for determining exactly which checks will run on which hosts and under what conditions. The most obvious and prevalent way they are used is in the config file, specifying criteria for NHC to use in deciding which checks it should run. While the 3 built-in mstr types (glob, regex, range) are usually sufficient for making that decision based solely on hostname/nodename, there are use cases for using other criteria, like nodegroups, Slurm partitions or states, etc., a few of which exist at Los Alamos.

To that end, NHC supports a fourth mstr type called "external." External matching is extremely useful but non-obvious, as it allows (or requires, depending on how you look at it!) the end-user/admin to define basically everything about how it works. Unfortunately, the current documentation hardly mentions them at all, except in passing, and has no examples or relevant details at all!

I need to add a lot more information to the README.md file. I'd also like to provide one or two simple examples -- perhaps one nodegroup-based and one Slurm-/state-based -- that are copy-and-paste-ready for users and admins to use, specifically the production team here at LANL!

See also #135 and my reply