Added wasm in the docs. (seanmonstar#1960)

Closes seanmonstar#1957

src/lib.rs

@@ -160,6 +160,12 @@
 //!   [`Identity`][Identity] type.
 //! - Various parts of TLS can also be configured or even disabled on the
 //!   `ClientBuilder`.
+//! 
+//! ## WASM
+//! The Client implementation automatically switches to the WASM one when the target_arch is wasm32,
+//! the usage is basically the same as the async api. Some of the features are disabled in wasm
+//! : [`tls`](tls) [`cookie`](cookie) [`blocking`](blocking).
+//! 
 //!
 //! ## Optional Features
 //!