update README: document optional features

Author: ratijas

README.md

@@ -59,9 +59,33 @@ Window {
 
 Requires Qt >= 5.8
 
+## Cargo features
+
+Cargo provides a way to enable (or disable default) optional [features](https://doc.rust-lang.org/cargo/reference/features.html).
+
+### `log`
+
+By default, Qt's logging system is not initialized, and messages from e.g. QML's `console.log` don't go anywhere.
+The "log" feature enables integration with [`log`](https://crates.io/crates/log) crate, the Rust logging facade.
+
+The feature is enabled by default. To activate it, execute the following code as early as possible in `main()`:
+
+```rust
+fn main() {
+    qmetaobject::log::init_qt_to_rust();
+    // don't forget to set up env_logger or any other logging backend.
+}
+```
+
+### `chrono_qdatetime`
+
+Enables interoperability of `QDate` and `QTime` with Rust [`chrono`](https://crates.io/crates/chrono) package.
+
+This feature is disabled by default.
+
 ## What if a binding for the Qt C++ API you want to use is missing?
 
-It is quite likely that you would like to call a particular Qt function wich is not wrapped by
+It is quite likely that you would like to call a particular Qt function which is not wrapped by
 this crate.
 
 In this case, it is always possible to access C++ directly from your rust code using the cpp! macro.