Add TUF roadmap (#202)

The roadmap was copied from a Hackmd created during the
TUF community meeting

Signed-off-by: Marina Moore <[email protected]>

Roadmap.md

@@ -0,0 +1,37 @@
+# TUF Roadmap
+
+### Housekeeping
+
+Which of these should block releases?
+* Convert detailed client workflow to use sub-procedures
+    * Goal: less repetition
+    * Goal: easy to implement and map to the spec requirements (checklist like)
+* Update specification tooling to do date and version bumping in GHA
+* Implementations should catch up with spec-1.x.y
+    * Resolution: implementations should explicitly list what they support
+* Should implementations refactor their code before implementing new TAPs?
+    * Resolution: up to implementations to define what they want to do
+
+## Goals for upcoming releases
+
+### Minor releases
+
+* [TAP 4: Multiple repository consensus on trusted targets](https://github.com/theupdateframework/taps/blob/master/tap4.md) (likely to be much easier if the client-workflow -> sub-procedures change is done first)
+* [TAP 15: Succinct hashed bin delegations](https://github.com/theupdateframework/taps/blob/master/tap15.md) (needs implementation & approval)
+* [TAP 13: User Selection of the Top-Level Target Files Through Mapping Metadata](https://github.com/theupdateframework/taps/pull/118) (needs accepting as draft, implementation & approval)
+* [TAP 16: Snapshot Merkle trees](https://github.com/theupdateframework/taps/blob/master/tap16.md) (needs implementation & approval)
+
+### Major releases
+
+* [TAP 3: Multi-role delegations](https://github.com/theupdateframework/taps/blob/master/tap3.md) (also likely to be easier if client-workflow refactor done first)
+* [TAP 8: Key rotation and explicit self-revocation](https://github.com/theupdateframework/taps/blob/master/tap8.md) (needs implementation & approval)
+* [TAP 14: Managing TUF versions](https://github.com/theupdateframework/taps/blob/master/tap14.md) (needs implementation & approval)
+* [TAP X: DSSE](https://github.com/theupdateframework/taps/pull/138) (needs accepting as draft, implementation and approval)
+
+### Timeline (2022)
+* Specification refactor by June
+* TAP 15 Accepted & in reference implementation by June
+* TAP 15 in the spec by end of year
+* TAP 3 & 4, work on implementations
+* Implement TAP 13 in go-tuf
+* All implementations should catch up to 1.x.y by $MONTH and $DATE?