Help: Add release note for fileapi "toolchains" object

This release note was accidentally left out of commit f72bb2ee0d (Help:
Add documentation for "toolchains" object kind, 2021-01-07).

Help/release/dev/fileapi-toolchains.rst

@@ -0,0 +1,5 @@
+fileapi-toolchains
+------------------
+
+* The :manual:`cmake-file-api(7)` gained a new "toolchains" object
+  kind that describes the compiler used for each enabled language.