ROADMAP.md: Add high level project roadmap

This documents the libp2p short and long term roadmap based on
previous work (e.g. see [libp2p/libp2p/60], [ipfs/roadmap/18],
[libp2p/specs/archive]).

[libp2p/libp2p/60]: libp2p/libp2p#60
[ipfs/roadmap/18]: ipfs/roadmap#18
[libp2p/specs/archive]: https://github.com/libp2p/specs/tree/master/_archive

README.md

@@ -25,8 +25,9 @@ For user-facing documentation, please see https://docs.libp2p.io
 
 In addition to describing the current state of libp2p, the specs repository
 serves as a coordination point and a venue to drive future developments in
-libp2p. To participate in the evolution of libp2p via the specs process, please
-see the [Contributions section](#contributions).
+libp2p. For the short and long term roadmap see [./ROADMAP.md]. To participate
+in the evolution of libp2p via the specs process, please see the [Contributions
+section](#contributions).
 
 ## Status