Docs: Add remote execution architecture documentation.

See merge request !923

Angelos Evripiotis authored Nov 26, 2018 and Angelos Evripiotis committed Nov 26, 2018

Update the section on adding tests, to fulfill these broad points:

- Mandate end-to-end testing, with rationale.
- Mention internal APIs as endpoints for testing.
- Warn against pitfalls of unit-testing.

It's more text than I would have liked, perhaps in later work we'll be
able to say it with less.

This change integrates feedback from the mailing list
'Guidance on Unit Tests' thread:
https://mail.gnome.org/archives/buildstream-list/2018-November/msg00045.html