add Nelson memo to open data section

[sformel-usgs]

Thank you for this very useful resource! I was browsing and noticed that the relatively recent Nelson memo isn't part of the open data section yet. But it's driving a lot of conversation and action in the data management world right now. Before I write a PR, just checking to see if anyone else is working on that yet?

[krusynth]

Please feel free to add it! I will admit that I do not do as much work on Open Data as I used to, so the site is likely not as up to date there as it could be. I hear there’s a new NAP in the works too.