doc: add arbitrary_metadata.md decision

doc/decisions/README.md

@@ -41,6 +41,7 @@ section here.
 - [Base Name](base_name.md)
 - [Characters](characters.md)
 - [Holes and Non-leaf values](holes.md)
+- [Arbitrary Meta-Data](arbitrary_metadata.md)
 - [Capabilities](capabilities.md)
 - [Array](array.md)
 - [Boolean](boolean.md)

doc/decisions/arbitrary_metadata.md

@@ -0,0 +1,35 @@
+# Arbitrary Metadata
+
+## Problem
+
+To make storage-plugins suitable for `spec` they need to be able to store
+all the meta-data as specified in [METADATA.ini](/doc/METADATA.ini).
+Most file formats do not have support for that.
+
+## Constraints
+
+## Assumptions
+
+## Considered Alternatives
+
+- store metadata in the comments like the `ini` plugin:
+  this exposes internal meta-data into the comments and
+  can drastically affect the readability of a storage file.
+  Comments should never be touched by a parser.
+- 
+
+## Decision
+
+Use different storage plugins, or plugins with different configurations,
+for the `spec` namespace:
+
+- `ni`
+- TOML with `meta` configuration
+
+## Rationale
+
+## Implications
+
+## Related Decisions
+
+## Notes