Update README (#74)

Manually validated build, see [build #520](https://travis-ci.org/algolia/algoliasearch-client-android/builds/127214968).

Author: algoliareadmebot

README.md

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+*Note: If you were using **version 2.x** of our Android client, read the [migration guide to version 3.x](https://github.com/algolia/algoliasearch-client-android/wiki/Migration-guide-to-version-3.x).*
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