doc/install: document system requirements for the FreeBSD/ARM port

R=golang-dev, r, mikioh.mikioh
CC=golang-dev
https://golang.org/cl/6816080

doc/install.html

@@ -40,7 +40,7 @@ proceeding. If your OS or architecture is not on the list, it's possible that
 <th align="middle">Notes</th>
 </tr>
 <tr><td colspan="3"><hr></td></tr>
-<tr><td>FreeBSD 7 or later</td> <td>amd64, 386</td> <td>Debian GNU/kFreeBSD not supported</td></tr>
+<tr><td>FreeBSD 7 or later</td> <td>amd64, 386, arm</td> <td>Debian GNU/kFreeBSD not supported; FreeBSD/ARM needs FreeBSD 10 or later</td></tr>
 <tr><td>Linux 2.6.23 or later with glibc</td> <td>amd64, 386, arm</td> <td>CentOS/RHEL 5.x not supported; no binary distribution for ARM yet</td></tr>
 <tr><td>Mac OS X 10.6/10.7</td> <td>amd64, 386</td> <td>use the gcc<sup>&#8224;</sup> that comes with Xcode<sup>&#8225;</sup></td></tr>
 <tr><td>Windows 2000 or later</td> <td>amd64, 386</td> <td>use mingw gcc<sup>&#8224;</sup>; cygwin or msys is not needed</td></tr>