docs: document using custom nonce (#905)

see https://github.com/FormidableLabs/react-native-app-auth/pull/736/files

README.md

@@ -135,7 +135,7 @@ with optional overrides.
   - **token** - (`{ [key: string]: value }`) headers to be passed during token retrieval request.
   - **register** - (`{ [key: string]: value }`) headers to be passed during registration request.
 - **additionalHeaders** - (`{ [key: string]: value }`) _IOS_ you can specify additional headers to be passed for all authorize, refresh, and register requests.
-- **useNonce** - (`boolean`) (default: true) optionally allows not sending the nonce parameter, to support non-compliant providers
+- **useNonce** - (`boolean`) (default: true) optionally allows not sending the nonce parameter, to support non-compliant providers. To specify custom nonce, provide it in `additionalParameters` under the `nonce` key.
 - **usePKCE** - (`boolean`) (default: true) optionally allows not sending the code_challenge parameter and skipping PKCE code verification, to support non-compliant providers.
 - **skipCodeExchange** - (`boolean`) (default: false) just return the authorization response, instead of automatically exchanging the authorization code. This is useful if this exchange needs to be done manually (not client-side)
 - **iosCustomBrowser** - (`string`) (default: undefined) _IOS_ override the used browser for authorization, used to open an external browser. If no value is provided, the `SFAuthenticationSession` or `SFSafariViewController` are used.