docs: Add example for Mutations to README (#197)

README.md

@@ -169,6 +169,42 @@ async function main() {
 main().catch((error) => console.error(error))
 ```
 
+### Mutations
+
+```js
+import { GraphQLClient, gql } from 'graphql-request'
+
+async function main() {
+  const endpoint = 'https://api.graph.cool/simple/v1/cixos23120m0n0173veiiwrjr'
+
+  const graphQLClient = new GraphQLClient(endpoint, {
+    headers: {
+      authorization: 'Bearer MY_TOKEN',
+    },
+  })
+
+  const mutation = gql`
+      mutation AddMovie($title: String!, $releaseDate: Int!) {
+        insert_movies_one(object: { title: $title, releaseDate: $releaseDate }) {
+            title
+            releaseDate
+        }
+    }
+  `
+
+  const variables = {
+    title: 'Inception',
+    releaseDate: 2010
+  }
+  const data = await graphQLClient.request(mutation, variables)
+
+  console.log(JSON.stringify(data, undefined, 2))
+}
+
+main().catch((error) => console.error(error))
+```
+
+
 [TypeScript Source](examples/using-variables.ts)
 
 ### Error handling