Add section about extension methods type parameters

Author: anatoliykmetyuk

docs/blog/_posts/2021-02-18-scala3-rc1.md

@@ -13,6 +13,34 @@ This release brings some last-minute polishings, clean-ups and changes before th
 Overall, more than [400 PRs](https://github.com/lampepfl/dotty/pulls?q=is%3Apr+is%3Aclosed+closed%3A%3E2020-12-02+sort%3Acomments-desc) were merged after the M3 release and until today! Read more below!
 
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+# Allow leading context parameters in extension methods
+Type parameters on extensions can now be combined with type parameters on the methods themselves. E.g.:
+
+```scala
+List(1, 2, 3).second[Int]
+extension [T](xs: List[T])
+   def def sumBy[B](f: A => B)(using Numeric[B]): B = ...
+```
+
+Type arguments matching method type parameters are passed as usual:
+
+```scala
+List("a", "bb", "ccc").sumBy[Int](_.length)
+```
+By contrast, type arguments matching type parameters following `extension` can be passed
+only if the method is referenced as a regular method:
+
+```scala
+List[String]("a", "bb", "ccc").sumBy(_.length)
+```
+
+Or, when passing both type arguments:
+
+```scala
+List[String]("a", "bb", "ccc").sumBy[Int](_.length)
+```
+
+For discussion, see [PR #10940](https://github.com/lampepfl/dotty/pull/10940). For more information about the extension methods, see [documentation](https://dotty.epfl.ch/docs/reference/contextual/extension-methods.html).
 
 # New `import` syntax
 The following are the changes to the `import` syntax made in this release.