Talk-android: Change readme in coherence with ios app

[Spartachetto]

A bit more "Brand" identity with talk-ios could be good.

A really simple proposal for the first part of the readme

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Nextcloud Talk Android app

This is the Android app for the Nextcloud Talk app.

Prerequisites

• Nexcloud instance with Nextcloud Talk app installed (Latest master)
• .....

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In addition it could be useful that each of the two apps cites the other.
Something like: "We are working also on the ios version of this app!"

[mario]

Will talk with @Ivansss about this, but priority is to get the apps running. Thanks for the note! :)

[mario]

Changing this a bit as we speak.