Add known issues section

Author: anatoliykmetyuk

docs/blog/_posts/2020-11-09-scala3-m1.md

@@ -76,6 +76,8 @@ During the last months, a considerable amount of effort went into investigating
 - Semanticdb usability enhancements [#9768](https://github.com/lampepfl/dotty/pull/9768)
 - Optimize core and frontend [#9867](https://github.com/lampepfl/dotty/pull/9867)
 
+# Known issues
+This release of Scala 3 doesn't work on JDK 14 because of a regression fixed in [#10135](https://github.com/lampepfl/dotty/pull/10135). JDK 15 doesn't work either because of [scala/bug#12172](https://github.com/scala/bug/issues/12172) which will be fixed in the new scala-library release.
 
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