docs(website): add parallel docs

Author: christianalfoni

packages/overmind-website/api/operators.md

@@ -56,6 +56,13 @@ You are only allowed to change the state in the mutate operator.
 h(Example, { name: "api/operators_operator_mutate" })
 ```
 
+## parallel
+Will run every operator and wait for all of them to finish before moving on. Works like *Promise.all*.
+
+```marksy
+h(Example, { name: "api/operators_operator_parallel" })
+```
+
 ## pipe
 The pipe is an operator in itself. Use it to compose other operators and pipes.
 

packages/overmind-website/examples/api/operators_operator_parallel.ts

@@ -0,0 +1,41 @@
+export default (ts) =>
+  ts
+    ? [
+        {
+          code: `
+import { Operator, pipe, mutate } from 'overmind'
+import { Item } from './state'
+
+export const loadSomeData: Operator<void, void> = pipe(...)
+export const loadSomeMoreData: Operator<void, void> = pipe(...)
+export const manageAllData = mutate(...)
+
+export const openItem: Operator<void, void> = pipe(
+  parallel([
+    loadSomeData,
+    loadSomeMoreData
+  ]),
+  manageAllData
+)
+`,
+        },
+      ]
+    : [
+        {
+          code: `
+import { pipe, mutate } from 'overmind'
+
+export const loadSomeData = pipe(...)
+export const loadSomeMoreData = pipe(...)
+export const manageAllData = mutate(...)
+
+export const openItem = pipe(
+  parallel([
+    loadSomeData,
+    loadSomeMoreData
+  ]),
+  manageAllData
+)
+`,
+        },
+      ]