Readme should mention that appAuthRedirectScheme must be lowercased for application be able to catch authorization redirect

[Dergash]

Description

I was working on migrating existing iOS application to react-native and encountered a problem which took some time for me to debug, so I hope this small readme update can save time for people in similar situation.

The problem was that I had existing redirectUrl specified in uppercase like this:

export const AUTH_CONFIG = {
  // ...
  redirectUrl: `COMPANY://audits`,
}

Following the docs for react-native-app-auth I have to assign appAuthRedirectScheme as a COMPANY part of the redirectUrl:

The scheme is the beginning of your OAuth Redirect URL, up to the scheme separator (:) character. E.g. if your redirect uri
is com.myapp://oauth, then the url scheme will is com.myapp.

But by following this instructions my application failed to catch authorization redirect from sso page to my application.
In fact, although redirectUrl is case-insensitive, appAuthRedirectScheme is not: it is a placeholder variable for <data android:scheme /> element in AppAuth-Android RedirectUriReceiverActivity, and the value of this tag should be lowercased according to official Android guide to app manifest.

android {
    defaultConfig {
        manifestPlaceholders = [
               // 'appAuthRedirectScheme': 'COMPANY' << incorrect
               'appAuthRedirectScheme': 'company' // << correct
        ]
    }

There are existing issues mentioning this problem in other libraries using net.openid:appauth (e.g. openid/AppAuth-Android#533 (comment), MaikuB/flutter_appauth#127 (comment) or stackoverflow), but it's very hard to catch without knowing how appAuthRedirectScheme is used internally.

[kadikraman]

Thanks for adding this!