x/website: update README.md
x/website: update README.md

The README in the root is being updated to let users know that this
website is under development and where they should direct their CLs.

Change-Id: I7355e1103e2ac202061db20b0c55e1cf6b1b95f6
Reviewed-on: https://go-review.googlesource.com/c/156957
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/website,golang/website,golang/website,golang/website,golang/website,golang/website,golang/website