[Critical] Toast not visible on production if your app enabled proguard

Package documentation should mention this, if an app use proguard and don't add this rule, then Toasts will only work on debug and not on release mode.

```
-keep class io.github.ponnamkarthik.toast.** { *; }
```

I didn't know if a package can automatically add missing stuff inside proguard file, so that's why i ask to mention that in doc.