Add section for metadata standards & compliance

[sadielbartholomew]

Hi there. This is a very useful listing but I feel there is a major omission, namely standards & conventions used widely across, and/or designed for, the Earth (Atmospheric, Ocean, and Climate as per your list scope) Science, notably the CF Conventions & COARDS.

I feel that advertising these here would be very beneficial, especially the CF Conventions which I think have significant influence on many software tools listed (& in fact is already referenced in the Iris item of the listing). There are further related tools that would in turn be beneficial to add, for example compliance checkers, e.g. as in this separate listing of tools on the CF Conventions site.

For the new section & sub-section structure I would suggest something like (using placeholder text for the links, which would be revised to be more descriptive & appropriate):

Metadata Standards
│
└───CF Conventions
│   │   [website & overview](https://cfconventions.org/index.html)
│   │   [latest canonical document/definition](https://cfconventions.org/latest.html)
│   │   ...
│   │
│   └───CF Compliance Checking
│       │   <checkers listing e.g. those under https://cfconventions.org/software.html)>
│   
└───COARDS
│   │   [latest canonical document/definition](https://ferret.pmel.noaa.gov/Ferret/documentation/coards-netcdf-conventions)
│   │   ... 
│
└─── ...

Or perhaps the standards and compliance checking tools can be listed separately under the top-level heading instead of being nested by standard.

I thought I should check if this would be desirable here in an Issue in the first instance, but if there is consensus among the repo owners for this or something similar (in which case please suggest how you may like to adapt it) I am happy to populate a PR to get with the necessary items & links to add the overall section into the listing. Thanks.