added 'AuthenticationController' file to handle the user input in registration process

server/src/controllers/AuthenticationController.js

-module.exports={
-    register(req, res){
+const {User}= require('../models')
+
 
-        res.send({
-        message: 'hello ${req.body.email} ! Your user was registered! Have fun!'
+module.exports={
+    async register(req, res){
 
+        try{
+        const user= await User.create(req.body)
+        res.send(user.toJSON())
+        }catch (err){
+            res.status(400).send({
+                error: 'This email account is already in use.'
             })
         }
+      
+  }
 }
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