Document ADR 0008 about unrecognized fields

Even though, this ADR documents something already implied in the TUF
spec in [document formats](https://theupdateframework.github.io/specification/latest/#document-formats)
it seems better to document this decision clearly so that it could be
referenced and give an explanation why someone can load a metadata file
with additional unrecognized fields.

Signed-off-by: Martin Vrachev <mvrachev@vmware.com>

docs/adr/0008-accept-unrecognised-fields.md

@@ -0,0 +1,44 @@
+#  Accept metadata that includes unrecognised fields
+
+- Status: accepted
+- Date: 2021-04-08
+
+Technical Story: https://github.com/theupdateframework/tuf/issues/1266
+
+## Context and Problem Statement
+
+The current reference implementation will ignore unrecognized fields in a
+metadata file when loading it.
+This leads to the side effect that if you read a metadata file with unrecognized
+fields and imidiatly write it back to the disk, this file will be modified.
+
+Furthermore, some TAPs ([6](https://github.com/theupdateframework/taps/blob/master/tap6.md), [10](https://github.com/theupdateframework/taps/blob/master/tap10.md), [14](https://github.com/theupdateframework/taps/blob/master/tap14.md), [15](https://github.com/theupdateframework/taps/blob/master/tap15.md), and [16](https://github.com/theupdateframework/taps/blob/master/tap16.md)) are relying on that unrecognized
+fields will be accepted to introduce new fields to the specification without
+making the metadata invalid for older clients which don't recognize the field.
+
+## Decision Drivers
+- The TUF specification implies support for custom attribute-value fields
+See [Document formats](https://theupdateframework.github.io/specification/latest/#document-formats)
+- Backward compatibility
+- If we perform the following operations on a metadata file with no
+intermediate operations:
+1. read the metadata file
+2. write the metadata file back to the disk
+the checksum (the content) of the file must not be changed.
+- Flexibility to add new fields in the spec without adding breaking changes.
+
+## Considered Options
+- Ignore and drop unrecognized fields.
+- Ignore, but store unrecognized fields as an additional attribute.
+
+## Decision Outcome
+
+Chosen option: "Ignore, but store unrecognized fields as an additional
+attribute."
+The motivations for this decision are that the TUF specification already implies
+that we should accept unrecognized fields for backward compatibility and easier
+future extensibility.
+
+Additionally, it seems unacceptable to change a metadata file content just by
+reading and writing it back.
+