added user signup feature

front/src/api.js

@@ -27,3 +27,13 @@ export function postBook(book) {
     body: JSON.stringify(book.value)
   })
 }
+
+export function postSignup(user) {
+  return fetch(baseUrl + '/auth/signup', {
+    method: 'POST',
+    headers: {
+      'Content-Type': 'application/json'
+    },
+    body: JSON.stringify(user.value)
+  })
+}