[Docs] Added new article to add custom button to a form (#2364)

frappe/docs/user/en/guides/app-development/adding-custom-button-to-form.md

@@ -0,0 +1,27 @@
+To create a custom button on your form, you need to edit the javascript file associated to your doctype. For example, If you want to add a custom button to User form then you must edit `user.js`.
+
+In this file, you need to write a new method `add_custom_button` which should add a button to your form.
+
+#### Function Signature for `add_custom_button(...)`
+    frm.add_custom_button(__(buttonName), function(){
+      //perform desired action such as routing to new form or fetching etc.
+    }, __(groupName));
+
+#### Example-1: Adding a button to User form
+We should edit `frappe\core\doctype\user\user.js`
+
+    frappe.ui.form.on('User', {
+    	refresh: function(frm) {
+        ...
+          frm.add_custom_button(__('Get User Email ID'), function(){
+            frappe.msgprint(frm.doc.email);
+        }, __("Utilities"));
+        ...
+      }
+    });
+
+You should be seeing a button on user form as shown below,
+
+![add_custom_button_screenshot](/assets/img/app-development/add_custom_button.png)
+
+<!-- markdown -->