Clarify that ruby-install does not support installing from rolling releases.

* This is due to ruby-install using ruby-versions to verify the checksums of a
  downloaded release. Since rolling releases are constantly being updated,
  oftenly daily, it would be very difficult to maintain an up-to-date list of
  checksums to verify against.
* It is a better long-term strategy, for both ruby-install and end-users,
  to encourage Ruby implementations to release stable versions and to
  support installing those.

Author: postmodern

README.md

@@ -32,7 +32,7 @@ Installs [Ruby], [JRuby], [Rubinius], [TruffleRuby] (native / GraalVM), or
 
 * Does not require updating every time a new Ruby version comes out.
 * Does not require recipes for each individual Ruby version or configuration.
-* Does not support installing trunk/HEAD.
+* Does not support installing trunk/HEAD or nightly rolling releases.
 * Does not support installing unsupported/unmaintained versions of Ruby.
 
 ## Requirements