doc: update to add ref to supported platforms

Update README.md to be consistent with changes made
to earlier branches.

PR-URL: https://github.com/nodejs/node/pull/12931
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: MichaëZasso <targos@protonmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>

README.md

@@ -159,7 +159,8 @@ the binary verification command above.
 ## Building Node.js
 
 See [BUILDING.md](BUILDING.md) for instructions on how to build
-Node.js from source.
+Node.js from source. The document also contains a list of
+officially supported platforms.
 
 ## Security