add words about version-negotiation

Overall Plan.rst

@@ -98,3 +98,16 @@
       #. Set the application up to accept `ps-pair:` URLs
       #. Parse the code out of such an URL (instead of user typing it in)
       #. Hard-code all other parameters (AppID, wormhole mailbox address, version, ..)
+
+   #. Extend the Android app to reject improper version negotiation
+
+      The deliverable is after the "support receiving a read-cap" milestone.
+      The magic-wormhole protocol understands the "app-versions" negotiation properly.
+
+      #. Fix the Haskell magic-wormhole library to understand "app-versions" (currently it "parses" them but throws them away / ignores it).
+      #. Fix the pairing protocol to ask for proper "app-versions" from ^
+      #. Fix the pairing protocol to send proper "app-versions" to the peer
+      #. Fail on incorrect or incompatible "app-versions"
+
+      Note that this is "very nice to have" for proper interoperability, but not strictly necessary for a first release.
+      Risk: before this is implemented, any subsequent release of anything in the stack that speaks a newer pairing protocol may break this application.