Add features for docs.rs build

rust-bindings/rust/Cargo.toml

@@ -77,3 +77,6 @@ v2018_6 = ["v2018_5", "ostree-sys/v2018_6"]
 v2018_7 = ["v2018_6", "ostree-sys/v2018_7"]
 v2018_9 = ["v2018_7", "ostree-sys/v2018_9"]
 v2019_2 = ["v2018_9", "ostree-sys/v2019_2"]
+
+["package.metadata.docs.rs"]
+features = ["dox", "futures"]