Update readme

[Alex009]

• logo (?)
• description with goals
• gif with clear example in action
• features - camera capture, gallery image/video pick, coroutines based api, file picker, video player common controller
• simple usage (capture photo from camera, select video from gallery, show video in preview)
• installation steps
• detailed usage
• how to contribute
• license block

[RezMike]

Current Usage section in readme is outdated - the API of the library has changed.

[Tetraquark]

Also need to add info about required maven repository url in root build.gradle

allprojects {
    repositories {
        // ...
        maven { url = uri("https://dl.bintray.com/lukaville/maven") }
    }