Add license badge to README

[schalkms]

No description provided.

[3flex]

No issue with the PR itself, but does FOSSA actually report anything useful for this repo?

It says "FOSSA does not fully support this environment yet" when clicking the link, and it doesn't report the license for any dependencies (which I thought was the idea with FOSSA). All it can do right now is report that detekt uses Apache 2.0 license, but that's clear from the license file and also shown in top right of repo homepage on Github:

[schalkms]

As it looks to me FOSSA doesn't scan the Gradle Kotlin build script, because it doesn't support it.
I remember that there was a license obligation problem with the detekt sonar plugin.
That's the reason for having the FOSSA scan included in all detekt-* projects.
At the moment it's not very useful, I agree. However, when there's support for the Gradle Kotlin DSL the build automatically fails, when something is wrong with the licensing stuff.