doc: document linux/ppc64 kernel requirement (2.6.37 or later)

Fixes #13269.

Change-Id: I960d1825bda9d8873c2a9005872c45e4c7d30111
Reviewed-on: https://go-review.googlesource.com/18339Reviewed-by: Ian Lance Taylor <iant@golang.org>

doc/install-source.html

@@ -93,6 +93,17 @@ The full set of supported combinations is listed in the discussion of
 <a href="#environment">environment variables</a> below.
 </p>
 
+<p>
+See the main installation page for the <a href="/doc/install#requirements">overall system requirements</a>.
+The following additional constraints apply to systems that can be built only from source:
+</p>
+
+<ul>
+<li>For Linux on PowerPC 64-bit, the minimum supported kernel version is 2.6.37, meaning that
+Go does not support CentOS 6 on these systems.
+</li>
+</ul>
+
 </div>
 
 <h2 id="go14">Install Go compiler binaries</h2>