Add secret key to environment variables, update readme

server/.env

@@ -1,3 +1,4 @@
 TZ=UTC
 BASE_URL=http://localhost:1337
 DATABASE_URL=postgresql://localhost/planka
+SECRET_KEY=notsecretkey

server/config/env/production.js

@@ -222,6 +222,8 @@ module.exports = {
       // secure: true,
       maxAge: 24 * 60 * 60 * 1000,  // 24 hours
     },
+
+    secret: process.env.SECRET_KEY,
   },
 
   /**************************************************************************