crypto/internal/boring: add dev.boringcrypto README.md text

crypto/internal/boring: add dev.boringcrypto README.md text

Add the text from dev.boringcrypto's README making clear what
this code is and that it is unsupported.

Change-Id: Ie49e8ccff10436f5d27ed422f159b4899193c9a5
Reviewed-on: https://go-review.googlesource.com/c/go/+/460515
Reviewed-by: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>
Run-TryBot: Russ Cox <rsc@golang.org>