add info how to migrate to v2

## Android Library v2

Starting from 01.10.2019 we will not actively develop our old library (v1), but maintain it until 01.10.2021 with bug fixes.

v2 is using [OkHTTP](https://square.github.io/okhttp) and [DAV4jvm](https://gitlab.com/bitfireAT/dav4jvm) by [BitfireAT](https://www.bitfire.at/).

Needed changes for projects using this library are:

- change build.gradle

  - add to android {…}: compileOptions {

  ```

        sourceCompatibility JavaVersion.VERSION_1_8

        targetCompatibility JavaVersion.VERSION_1_8

  ```

    }

  -  add to dependencies {…}:

  ```

     implementation "commons-httpclient:commons-httpclient:3.1@jar" // remove after entire switch to lib v2

  ``` 

## Use Library

In the repository it is not only the library project but also the example project "sample_client";