Release RC for 0.13.0

[Jolanrensen]

We have enough new content to start releasing version 0.13.0 of DF.
To prevent releasing some overlooked bugs or missing documentation, we want to release a release candidate first.

[Jolanrensen]

Depends on #590

[Jolanrensen]

And #550

[Jolanrensen]

Need to check which steps of https://github.com/Kotlin/dataframe/blob/master/RELEASE_CHECK_LIST.md apply for RCs

[Jolanrensen]

Let's follow the naming convention: 0.x.y-RCz
This will allow us to have multiple release candidates, but not more than 9.

The first will be called 0.13.0-RC1

[Jolanrensen]

Well... Due to a configuration mistake, 0.13.0 was released.
We'll try to release an RC again for the next version :)