Add prior file and mention of COSMOS SEDs

[sschmidt23]

I'm going to add the 31 SED COSMOS set from LePhare, I can include the equivalent of the HDFN prior for that template set, along with a mention in the README that says that those templates are available (though the SEDs will actually be in rail_base)

• I have described the purpose of the suggested change, specifying what I need the enhancement to accomplish, i.e. what problem it solves.
• I have included any relevant links, screenshots, environment information, and data relevant to implementing the requested feature, as well as pseudocode for how I want to access the new functionality.
• If I have ideas for how the new feature could be implemented, I have provided explanations and/or pseudocode and/or task lists for the steps.

[sschmidt23]

Actually, prior is also in rail_base, so this will just be a modification of the README.md