add kotlin example to readme

Problem

No kotlin example in the readme

Solution

Add kotlin example

Test(s) added

only my eyes

Paired with

Nobody

CI build failed cos

Errors found:

/storage/jenkins/workspace/no-player-prb/core/build.gradle:62: Error: A newer version of com.google.android.exoplayer:exoplayer than 2.9.6 is available: 2.10.1 [GradleDependency]
    implementation 'com.google.android.exoplayer:exoplayer:2.9.6'
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/storage/jenkins/workspace/no-player-prb/core/build.gradle:67: Error: A newer version of org.mockito:mockito-core than 2.27.0 is available: 2.28.2 [GradleDependency]
    testImplementation 'org.mockito:mockito-core:2.27.0'
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

not really a part of this PR

I agree. It's because we haven't updated master in a long time. We need to remove MediaPlayer from the code and do some tidying up after we added the AdsLoader.