doc/go1.8: document mips port requirements

Fixes #18140.

Change-Id: I527719576bc7198c79bb8bc51d258a71b2d5b296
Reviewed-on: https://go-review.googlesource.com/33853Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

doc/go1.8.html

@@ -77,7 +77,9 @@ func example() {
 <p>
 Go now supports 32-bit MIPS on Linux for both big-endian
 (<code>linux/mips</code>) and little-endian machines
-(<code>linux/mipsle</code>).
+(<code>linux/mipsle</code>) that implement the MIPS32r1 instruction set with FPU
+or kernel FPU emulation. Note that many common MIPS-based routers lack an FPU and
+have firmware that doesn't enable kernel FPU emulation; Go won't run on such machines.
 </p>
 
 <p>