Update SECURITY.md

Author: leerob

SECURITY.md

@@ -1,5 +1,9 @@
-# Security
+# Reporting Security Issues
 
-Email our security team (`[email protected]`) to disclose any security vulnerabilities.
+If you believe you have found a security vulnerability in Next.js, we encourage you to let us know right away.
+
+We will investigate all legitimate reports and do our best to quickly fix the problem.
+
+Email `[email protected]` to disclose any security vulnerabilities.
 
 https://vercel.com/security