Add build instructions where necessary (like Updatenotifications app which needs make)

[jancborchardt]

Because currently no one knows it needs make.

cc @skjnldsv @MorrisJobke as discussed. We should have a readme in the app folder, or have it in the normal readme. cc @blizzz

[skjnldsv]

Closing in favour of https://github.com/nextcloud/documentation/pulls?q=is%3Apr+is%3Aopen+sort%3Aupdated-desc+author%3Askjnldsv

[jancborchardt]

So is this covered by the "Building frontend code" part in the readme? https://github.com/nextcloud/server#building-front-end-code-

Cause it should be in the readme directly too for quick access, not only in the documentation.