doc: note that EoL platforms are not supported

Add a note to clarify that any platform that is EoL will not be
supported by Node.js.

PR-URL: nodejs#12672
Fixes: nodejs/build#688
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Rod Vagg <rod@vagg.org>

BUILDING.md

@@ -33,6 +33,10 @@ Support is divided into three tiers:
 
 ### Supported platforms
 
+The community does not build or test against end of life distributions (EoL).
+Thus we do not recommend that you use Node on end of life or unsupported platforms
+in production.
+
 |  System      | Support type | Version                          | Architectures        | Notes            |
 |--------------|--------------|----------------------------------|----------------------|------------------|
 | GNU/Linux    | Tier 1       | kernel >= 2.6.32, glibc >= 2.12  | x86, x64, arm, arm64 |                  |