Add documentation

Author: armano2

docs/rules/no-async-in-computed-properties.md

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+# Check if there are no side effects inside computed properties (no-async-in-computed-properties)
+
+Check if there are no side effects inside computed properties
+
+
+## :book: Rule Details
+
+This rule tries to check if there are no side effects inside computed properties.
+
+This rule aims to...
+
+:-1: Examples of **incorrect** code for this rule:
+
+```js
+computed: {
+  foo: async function () {
+    return await someFunc()
+  }
+}
+```
+
+```js
+computed: {
+  foo () {
+    return new Promise((resolve, reject) => {})
+  }
+}
+```
+
+```js
+computed: {
+  foo () {
+    return fetch(url).then(response => {})
+  }
+}
+```
+
+:+1: Examples of **correct** code for this rule:
+
+```js
+computed: {
+  foo () {
+    return this.a + this.b
+  }
+}
+```
+
+## :wrench: Options
+
+Nothing.