iOS 15+ mention Substitute in Sileo

[madaster97]

Addressing issue 112, explicitly mention Substitute (Sileo's replacement for Cydia Substrate) in the install documentation.

I learned about this replacement in a discord chat with palera1n folks.

[P5-2005]

iirc sileo on rootless cannot use substitute, work only on rootfull, even with ellekit(like substrate) not working(on my end), also there is https://github.com/NyaMisty/ssl-kill-switch3 they add rootless support for arm64e(thanks misty), didnt worked as expected from my end too...