Document SCSS usage

[MorrisJobke]

@skjnldsv Can you add a note about SCSS usage in apps in the dev docs?

cc @raimund-schluessler

ref nextcloud/server#3530 (comment)

[skjnldsv]

Adding this to "The list"! 😭

[raimund-schluessler]

Would be great to know how I am supposed to include the generated css file from the appdata directory into the app. 😄 A simple style('appname', 'style'); does obviously not work anymore.

Edit: Looking at the code, I realised it is supposed to work, but it does not for me. I will investigate and report an issue if necessary.

[raimund-schluessler]

I guess this is a bug: see nextcloud/server#3606