Add UI/UX design principles to contributing documentation

[See original issue on GitLab](https://gitlab.com/BuildStream/buildstream/-/issues/1139)
In GitLab by [[Gitlab user @tlater]](https://gitlab.com/tlater) on Sep 16, 2019, 13:52

## Background

[//]: # (Provide a background or the root/source that justifies this task or action.)

[[Gitlab user @cs-shadow]](https://gitlab.com/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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[//]: # (To review information about possible relevant labels for this issue please view the list of labels: https://gitlab.com/BuildStream/buildstream/labels)

Start with the contributing documentation and read the linked UI and UX design principles from the mailing list. Document the agreed principles in the contributing documentation, with completion confirmed when both source discussions are represented there.

Written by the indexing model from the issue text.