Rewrite for a PostgreSQL backend

cmd/discosrv/README.md

@@ -5,7 +5,24 @@ discosrv
 
 This is the global discovery server for the `syncthing` project.
 
-`go get github.com/syncthing/discosrv`
+To get it, run `go get github.com/syncthing/discosrv` or download the
+[latest build](http://build.syncthing.net/job/discosrv/lastSuccessfulBuild/artifact/)
+from the build server.
 
-Or download the latest [Linux build](http://build.syncthing.net/job/discosrv/lastSuccessfulBuild/artifact/).
+Usage
+-----
 
+The discovery server requires a postgresql backend server. You will need
+to create a database and a user with permissions to create tables in it.
+Set the database URL in the environment variable `DISCOSRV_DB` before
+starting discosrv.
+
+```bash
+$ export DISCOSRV_DB="postgres://user:password@localhost/databasename"
+$ discosrv
+```
+
+The appropriate tables and indexes will be created at first startup. If
+it doesn't exit with an error, you're fine.
+
+See `discosrv -help` for other options.