Add a section about future and future migration

Author: anatoliykmetyuk

docs/blog/_posts/2021-02-17-scala3-rc1.md

@@ -107,7 +107,9 @@ As the warning suggests, now you should write `Foo.apply` instead of `Foo`. See
 # Settling on `scaladoc` as the documentation tool
 We have settled on using `scaladoc` as the documentation build tool. The obsolete `dotty-doc` (or `scala3-doc`) is removed in RC1. We have also removed all the Kotlin dependencies (Dokka, etc.) from scaladoc. For details, see [PR #11349](https://github.com/lampepfl/dotty/pull/11349). To read more about `scaladoc`, see [documentation](https://dotty.epfl.ch/docs/usage/scaladoc/index.html)
 
-
+# Use `future` and `future-migration` to specify language versions after 3.0 in `-source`
+[PR #11355](https://github.com/lampepfl/dotty/pull/11355) changes the `-source` specifier for the Scala version(s) after 3.0 from `3.1` to `future`. I.e. it is now
+`-source future` and `-source future-migration` instead of `-source 3.1` and `-source 3.1-migration`. Language imports are changed analogously. The reason for the change is that we want to keep the possibility open to ship a `3.1` version that does not yet contain all the changes enabled under `-source future`.
 
 # Other language changes
 - Warn when matching against an opaque type [#10664](https://github.com/lampepfl/dotty/pull/10664)