cmd/compile: update README for rulegen

Update cmd/compile/internal/ssa/gen/README, suggest using go-1.13
and above for rulegen.

Change-Id: I5256d6f1bb4273b5eb7d5122a4364add52fc42c6
Reviewed-on: https://go-review.googlesource.com/c/go/+/192517Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/compile/internal/ssa/gen/README

@@ -3,5 +3,5 @@
 // license that can be found in the LICENSE file.
 
 This package generates opcode tables, rewrite rules, etc. for the ssa compiler.
-Run it with:
+Run it with go-1.13 (or above):
    go run *.go