docs: Update storage roadmap & contributing doc (#786)

* docs(roadmap): Update Storage section of roadmap

* docs(CONTRIBUTING): Add links to issue labels

CONTRIBUTING.md

@@ -7,8 +7,10 @@ potential improvements you want to make before sending a Pull Request.
 The [roadmap](roadmap.md) is probably the best place to find areas where help would
 most be appreciated.
 
-The Issue Tracker may contain items labelled **help wanted** or **good first issue**
-from time to time.
+The Issue Tracker may contain items labelled [**good first issue**][gfi] or [**help wanted**][hw] from time to time.
+
+[hw]: https://github.com/boardgameio/boardgame.io/issues?q=is%3Aissue+is%3Aopen+label%3A%22help+wanted%22+
+[gfi]: https://github.com/boardgameio/boardgame.io/issues?q=is%3Aissue+is%3Aopen+label%3A%22good+first+issue%22
 
 ## Pull Requests
 

roadmap.md

@@ -25,9 +25,11 @@ The issues below (and some others that are not in this document) are also availa
 
 - ##### Databases
 
-  - [x] add Mongo support
-  - [x] add Firebase support
-  - [ ] add Postgres support
+  We encourage developers to contribute third-party storage connectors, implementing
+  [the `StorageAPI.Async` interface](https://github.com/boardgameio/boardgame.io/blob/master/src/server/db/base.ts).
+
+  See the [Storage docs](https://boardgame.io/documentation/#/storage) for
+  examples and details of the backends currently available.
 
 - ##### Size / Performance