Add UI/UX design principles to contributing documentation

Background

@cs-shadow defined a couple of design principles on the mailing list which we agreed upon:

• UI: https://mail.gnome.org/archives/buildstream-list/2018-November/msg00106.html
• UX: https://mail.gnome.org/archives/buildstream-list/2018-December/msg00000.html

Task description

These design principles should be documented in the contributing documentation.

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