doc: add supported platforms list for v6.x

Original Commit Message:
  PR-URL: nodejs#11943
  Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
  Reviewed-By: James M Snell <jasnell@gmail.com>

Backport-Of: nodejs#11943

Author: mhdawson

BUILDING.md

@@ -39,7 +39,7 @@ Support is divided into three tiers:
 | GNU/Linux    | Tier 1       | kernel >= 2.6.18, glibc >= 2.5   | x86, x64, arm, arm64 |                  |
 | macOS        | Tier 1       | >= 10.10                         | x64                  |                  |
 | Windows      | Tier 1       | >= Windows 7 or >= Windows2008R2 | x86, x64             |                  |
-| SmartOS      | Tier 2       | >= 15 < 16.4                     | x86, x64             | see note1        |
+| SmartOS      | Tier 2       | >= 14 < 16.4                     | x86, x64             | see note1        |
 | FreeBSD      | Tier 2       | >= 10                            | x64                  |                  |
 | GNU/Linux    | Tier 2       | kernel >= 4.2.0, glibc >= 2.19   | ppc64be              |                  |
 | GNU/Linux    | Tier 2       | kernel >= 3.13.0, glibc >= 2.19  | ppc64le              |                  |
@@ -63,8 +63,8 @@ Depending on host platform, the selection of toolchains may vary.
 
 #### Unix
 
-* GCC 4.8.5 or newer
-* Clang 3.4.1 or newer
+* GCC 4.8 or newer
+* Clang 3.4 or newer
 
 #### Windows
 

README.md

@@ -132,7 +132,7 @@ 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 along with a list of officially supported platforms.
 
 ## Security