Add untrack()

docs/api/reactivity-utility.md

@@ -50,3 +50,35 @@ Runs a callback anytime a function scope is re-ran.
         return label
     end)
     ```
+
+## untrack()
+
+Gets the value of a source without reactively tracking it.
+
+- **Type**
+
+    ```lua
+    function untrack<T>(source: () -> T): T
+    ```
+
+- **Details**
+
+    Updates made to a source passed to `untrack()` will not cause updates to
+    anything depending on that source.
+
+- **Example**
+
+    ```lua
+    local a = source(0)
+    local b = source(0)
+
+    local sum = derive(function()
+        return a() + untrack(b)
+    end)
+
+    print(sum()) -- 0
+    b(1)
+    print(sum()) -- 0
+    a(1)
+    print(sum()) -- 2
+    ```