doc: add riscv64 info into platform list

Author: luyahan

BUILDING.md

@@ -109,6 +109,7 @@ platforms. This is true regardless of entries in the table below.
 | GNU/Linux        | armv6            | kernel >= 4.14, glibc >= 2.24   | Experimental                                    | Downgraded as of Node.js 12               |
 | GNU/Linux        | ppc64le >=power8 | kernel >= 3.10.0, glibc >= 2.17 | Tier 2                                          | e.g. Ubuntu 16.04[^1], EL 7[^2]           |
 | GNU/Linux        | s390x            | kernel >= 3.10.0, glibc >= 2.17 | Tier 2                                          | e.g. EL 7[^2]                             |
+| GNU/Linux        | riscv64          | kernel >= 3.10.0, glibc >= 2.17 | Experimental                                    |                                           |
 | Windows          | x64, x86 (WoW64) | >= Windows 10/Server 2016       | Tier 1                                          | [^4],[^5]                                 |
 | Windows          | x86 (native)     | >= Windows 10/Server 2016       | Tier 1 (running) / Experimental (compiling)[^6] |                                           |
 | Windows          | x64, x86         | Windows 8.1/Server 2012         | Experimental                                    |                                           |