Create a Wiki page with instructions how to install Signal from Github

[Tourniquet88]

Seeing as people have trouble sideloading the App on Desktops (which makes sense) it might be best to add a section to the Wiki to explain HOW to sideload on Mobile / PC and explaining that you can't cross update with store / Github version.

[KhaaL]

@Tourniquet88 do you have the instructions somewhere for how to sideload the app? I could write a page with instructions on the wiki as long as I know how to do it 😃

[golf1052]

😅 This isn't actually possible right now because my store key certificate is expired. I'm waiting on @Trolldemorted to send me the latest version.