Documentation: README.md: Usage: add Kotlin

[CharlesMAtkinson]

Based on my testing, the new information needs to be something like as follows.

If building with gradle, add this line to app/build.gradle.kts
implementation("de.cketti.mailto:email-intent-builder:2.0.0")

In the .kt file in which EmailIntentBuilder is called, need
import de.cketti.mailto.EmailIntentBuilder

Example function calling EmailIntentBuilder

    fun sendEmail(recipient: String, subject: String, body: String) {
        val success: Boolean = EmailIntentBuilder.from(this)
            .to(recipient)
            .subject(subject)
            .body(body)
            .start()
        if (!success) Log.e(TAG, "sendEmail: EmailIntentBuilder exception caught")
    }