Add api-catalog.yaml file (#1822)

Also added ARD008 to explain the decision. This is linked from
`mkdocs.yaml` and `docs/README.md`.

Co-authored-by: David Tuite <dtuite@gmail.com>

docs/architecture-decisions/adr008-default-catalog-file-name.md

@@ -0,0 +1,22 @@
+# ADR008: Default Catalog File Name
+
+## Background
+
+While the spec for the catalog file format is well described in
+[ADR002](./adr002-default-catalog-file-format.md), guidance was note provided as
+to the name of the catalog file.
+
+Following discussion in
+[Issue 1822](https://github.com/spotify/backstage/pull/1822#pullrequestreview-461253670),
+a decision was made.
+
+## Name
+
+The catalog file should be named
+
+```shell
+catalog-info.yaml
+```
+
+This name is a default, **not a requirement**. The catalog file will work with
+Backstage irregardless of its name.