Add substreamer into apps.md

[ghenry22]

Hi Everyone,

It's been a while, as previously requested, I have opened a new issue to get substreamer added back into the list of available apps for airsonic.

New website is up and running:
https://substreamerapp.com/

App is available for both Android and iOS with a huge update just released to both stores this week. I know I've been quiet for a while but I have been working on substreamer in the background and using the various betas myself and finally got everything together and properly released again!