doc/go1.7.html: add known issues section for FreeBSD crashes

Updates #16396

Change-Id: I7b4f85610e66f2c77c17cf8898cc41d81b2efc8c
Reviewed-on: https://go-review.googlesource.com/25283Reviewed-by: Chris Broadfoot <cbro@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>

doc/go1.7.html

@@ -102,6 +102,17 @@ POWER5 architecture.
 The OpenBSD port now requires OpenBSD 5.6 or later, for access to the <a href="http://man.openbsd.org/getentropy.2"><i>getentropy</i>(2)</a> system call.
 </p>
 
+<h3 id="known_issues">Known Issues</h3>
+
+<p>
+There are some instabilities on FreeBSD that are known but not understood.
+These can lead to program crashes in rare cases.
+See <a href="https://golang.org/issue/16136">issue 16136</a>,
+<a href="https://golang.org/issue/15658">issue 15658</a>,
+and <a href="https://golang.org/issue/16396">issue 16396</a>.
+Any help in solving these FreeBSD-specific issues would be appreciated.
+</p>
+
 <h2 id="tools">Tools</h2>
 
 <h3 id="cmd_asm">Assembler</h3>