Add documentation

docs/media/README-7.MD

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+# @welshman/store [![version](https://badgen.net/npm/v/@welshman/store)](https://npmjs.com/package/@welshman/store)
+
+Utilities for dealing with svelte stores when using welshman.
+
+```typescript
+import {Repository, NAMED_PEOPLE, NAMED_TOPICS, type TrustedEvent, readUserList, List} from '@welshman/util'
+import {deriveEventsMapped} from '@welshman/store'
+
+const repository = new Repository()
+
+// Create a svelte store that performantly maps matching events in the repository to List objects
+const lists = deriveEventsMapped<PublishedUserList>(repository, {
+  filters: [{kinds: [NAMED_PEOPLE, NAMED_TOPICS]}],
+  eventToItem: (event: TrustedEvent) => (event.tags.length > 1 ? readUserList(event) : null),
+  itemToEvent: (list: List) => list.event,
+})
+```