Address some feedback

Author: anatoliykmetyuk

docs/blog/_posts/2020-12-18-scala3-m3.md

@@ -15,6 +15,8 @@ In this article, you will find the most important changes of this release compar
 
 To read more about all things that surround the Scala 3 release in the next months, check out the "[Scala 3 - Crossing the finish line](https://www.scala-lang.org/blog/2020/12/15/scala-3-crossing-the-finish-line.html)" blog post on scala-lang.org
 
+You can try out the M3 version online via [Scastie](https://scastie.scala-lang.org/?target=dotty).
+
 <!--more-->
 # sbt plugin update
 We published a new version of the sbt plugin `sbt-dotty`, v0.5.0. Because of the changes in PR [#10607](https://github.com/lampepfl/dotty/pull/10607), this release of Scala 3 will not work with earlier versions of sbt-dotty. You will need to upgrade sbt-dotty to 0.5.0 to be able to use Scala 3.0.0-M3.
@@ -124,6 +126,8 @@ In the meantime, the compiler will emit warnings when trying to call those metho
   |            but it has unmatchable type opaques.IArray[Int] instead
 ```
 
+Note that the warnings are only active with language mode `3.1-migration` or higher - see the documentation on the [Language Versions](https://dotty.epfl.ch/docs/usage/language-versions.html) to learn how to enable it.
+
 You can read the discussion of this change in the [PR #10670](https://github.com/lampepfl/dotty/pull/10670). You can also read more about it in the [documentation](https://dotty.epfl.ch/docs/reference/other-new-features/matchable.html).
 
 # Tooling improvements
@@ -183,19 +187,13 @@ According to `git shortlog -sn --no-merges 3.0.0-M2..3.0.0-M3` these are:
 TODO
 ```
 
-If you want to get your hands dirty and contribute to Dotty, now is a good time to get involved!
-Head to our [Getting Started page for new contributors](https://dotty.epfl.ch/docs/contributing/getting-started.html),
-and have a look at some of the [good first issues](https://github.com/lampepfl/dotty/issues?q=is%3Aissue+is%3Aopen+label%3Aexp%3Anovice).
-They make perfect entry points into hacking on the compiler.
+If you want to get your hands dirty and contribute to Scala 3, head to our [Getting Started page for new contributors](https://dotty.epfl.ch/docs/contributing/getting-started.html) and read the docs on contributing. You are also very welcome to contact someone from our team to help you get started and see what currently we need help with.
 
 We are looking forward to having you join the team of contributors.
 
 ## Library authors: Join our community build
 
-Scala 3 has a set of community libraries that are built against every nightly
-snapshot. Currently, this includes shapeless, ScalaPB, algebra, scalatest, scopt and squants.
-Join our [community build](https://github.com/lampepfl/dotty/tree/master/community-build)
-to make sure that our regression suite includes your library.
+Scala 3 is regularly tested against a sample of libraries known as the "community build". You can add your library to the [community build](https://github.com/lampepfl/dotty/tree/master/community-build) by submitting a PR.
 
 [Scastie]: https://scastie.scala-lang.org/?target=dotty