Update downloads page at Debian Stretch section

[fadelkon]

Currently no references are given on how to "update manually" the dependency libsqlcipher. Please consider also to upload the library debfile to the code repository on Github.

Thanks

[csoler]

thx. I think we should also suggest to use the AppImage version on Debian, as it seems to work perfectly. Can you add this before l merge this in?

[fadelkon]

Done! 😃