docs(adr-001): add ADR for branch rename

This creates an Architecture Decision Records (ADRs) for #94 (closed).

If you want to learn more about ADRs you might find this article and this video helpful.

Fixes #100

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

+# 1. 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 stereo-types 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 histories
+of stories about the history of humans, legends and gods. This idea reflects
+very well what happens in a Git repository. Changes happen in branches (some
+teams tie them to _User Stories_), which are sometimes directly, sometimes
+loosely coupled to the main branch. Once they are finished the get added to the
+main branch, or in case of a rebase directly appended. The mental model of a big
+book of stories fits this process very well.
+
+Therefore the `master` branch is renamed to `saga` and becomes the main branch.
+`master` shall no longer be used.