I am building an app for saloon which includes services of that saloon. if user book the order only after login is it really necessary that we should add the skip log in page in the app .
I understand your question as follows (and maybe you can clarify the question to allow better answers):
Your app will list services of a "saloon", and allow clients to book these services. Booking is only possible after the client has logged in (to verify the client's authorizations, or to ensure the service is payed, or whatever other reason).
Your question is whether to allow usage of the service list (i.e., viewing the services) without logging in first.
I'd say, sure! Why should somebody register (which I assume is needed before logging in) if she does not know which services are offered (at what price)? So to explain why people should register/log in, they need information about the services first.
You can keep continue as guest option in the login page. User will be in the state of mind that he is viewing the services as guest and once he likes the services and places the order, he will be forced to login.