Disable default features for reqwest

Using `reqwest` with default-features enabled will enable the
`default-tls`, which in turn will enable the `native-tls` feautre which
will result in an attempt to compile against OpenSSL [0]. If one wants
to use `mastodon-async` without depending on OpenSSL, the default
features for `reqwest` must be disabled.

The default features for `mastodon-async` will enable
`reqwest/default-tls`, so the default behaviour won't change, but now it
is possible to declare the dependency on `mastodon-async` with default
features disabled and the `rustls-tls` feature enabled, allowing to
compile without a dependency on OpenSSL.

[0]: e02df1f448/Cargo.toml (L30)

Cargo.toml

@@ -75,6 +75,7 @@ features = ["serde"]
 [dependencies.reqwest]
 version = "0.11"
 features = ["multipart", "json", "stream"]
+default-features = false
 
 [dependencies.serde]
 version = "1"

README.md

@@ -25,6 +25,14 @@ Alternatively, run the following command:
 $ cargo add mastodon-async
 ~~~
 
+### Use Rustls instead of OpenSSL
+
+To use Rustls instead of OpenSSL for HTTPS request, define the dependency as follows
+
+```toml
+mastodon-async = { version = "1", default-features = false, features = ["rustls-tls"] }
+```
+
 ## A Note on Debugging
 This library offers structured logging. To get better information about bugs or
 how something is working, I recommend adding the femme crate as a dependency,