Adding CONTRIBUTING with descriptions and current state.

When an API exception is happening the exception should be caught in catch block is it?

`
fun invoke(
scope: CoroutineScope,
params: Params?,
onResult: (UseCaseResponse)
) {
val backgroundJob = scope.async {
run(params)
}
scope.launch {
backgroundJob.await().let {
try {

How about showing in README first how kotlin would do it's null mapping, something like this:

getCurrentUserId()
    ?.takeIf { it.isNotEmpty() && it != "Invalid Id" }
    ?.let { getCurrentUserFromDatabase(it) }
    ?.username
    ?.let { "Logged in user: $it" }
    .apply {
        if (this != null) {
            log(this)
        } else {
            log("No user to login!")