Add instructions for how to refresh the cache

[matildastevenson]

Possibly in a caching vignette, or in the main vignette

[daxkellie]

Caching was removed in previous version due to increasingly difficult problems integrating with upgrades that eventually led to a new query architecture in galah 2.0.0. However, galah 2.0.0 is much faster that previous versions and allows the ability to send queries on the server-side without holding up the R terminal 🥳

Closing for now while caching is no longer supported