add external store guide (#64)

docs/guides/external-store.md

@@ -0,0 +1,32 @@
+---
+layout: default
+title: Using Veles with external stores
+nav_order: 3
+parent: Guides
+---
+
+## Using external store sources
+
+You'll probably use some external state manager for your application, but you'll need to connect it first to convert necessary data to Veles' `state` which allows you to have reactivity. Right now there are no official bindings, but here is an example of a simple helper function to connect it to [Zustand](https://github.com/pmndrs/zustand):
+
+```tsx
+const NO_VALUE = "NO_VALUE";
+export function createStoreState<T>(selector: (state: ZustandStoreState) => T) {
+  let prevValue: T | string = NO_VALUE;
+  const initialValue = selector(store.getState());
+  return createState(initialValue, (setStoreValue) => {
+    const unsubscribe = store.subscribe((newState) => {
+      const newValue = selector(newState);
+
+      if (newValue !== prevValue) {
+        prevValue = newValue;
+        setStoreValue(newValue);
+      }
+    });
+
+    return unsubscribe;
+  });
+}
+```
+
+It has a simple comparator option to avoid unnecessary updates, but you expand it with a comparator to allow to refine it even further. Other state manager, like [Redux](https://redux.js.org/) should have a very similar implementation, they typically expose `subscribe` function and in the callback you'll need to update your state value.