docs(adr): add ADRs

They have been removed from the docs:
NordicSemiconductor/asset-tracker-cloud-docs@1db3320

adr/001-use-saga-as-the-main-branch.md

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+# ADR 001: Use saga as the name for the main branch
+
+Historically, Git and other software use terms such as `master/slave`, `whitelist/blacklist`, which are based on racial concepts.
+Their continued use maintains the racial stereotypes they depict.
+Better alternatives in meaning and technical correctness exist, like `leader/follower`, `blocklist/allowlist`.
+
+In the Nordic mythology, a `saga` is a long, continuous recollection of stories about the history of humans, legends, and Gods.
+The term `saga` reflects very well what happens in a Git repository.
+Changes happen in branches (some teams tie them to _User Stories_, which are sometimes directly or loosely coupled to the main branch).
+Once the changes are finalized, they get added to the main branch, or get appended in the case of a rebase.
+The mental model of a big book of stories fits this process very well.
+
+Therefore, the main branch in this project is named `saga`.
+`master` must not be used.

adr/README.md

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+# Architecture decision records
+
+This folder contains the architecture decision records (ADRs) for this project.
+
+To know more about ADRs, see [Documenting architecture decisions](http://thinkrelevance.com/blog/2011/11/15/documenting-architecture-decisions) and the video on [Communicating and documenting architectural decisions](https://www.youtube.com/watch?v=rwfXkSjFhzc).