cmd/vet/all/whitelist: add readme.txt

This was written for CL 27811,
but it got lost in the sea of new files.

Change-Id: I5c52cb23dda499b21a6bb32ed5c586779ccbc2f1
Reviewed-on: https://go-review.googlesource.com/28960Reviewed-by: Rob Pike <r@golang.org>

src/cmd/vet/all/whitelist/readme.txt

+This directory contains whitelists for vet complaints about the standard library and commands.
+They are line-based and unordered, although counts of duplicated lines matter.
+Each line matches vet's output, except that line numbers are removed to avoid churn.
+There are also os-, arch-, and bitwidth-specific whitelists.