Devise a tag/label system and index

[adele-morrison]

Often there are useful bits of code within recipes focused on showing something different. e.g. How to use the GibbSeaWater package, how to save intermediate output files etc.

We would love to have a system of add tags / labels to Recipes so people can find these specific useful pieces of code that aren't obvious just from the recipe name.

Ideas on how we could do this?