cmd/vet: add a README explaining the criteria for new checks

Correctness, Frequency, Precision.

Change-Id: I7f202c220aef8512d611dc04a4370b4a237f217c
Reviewed-on: https://go-review.googlesource.com/20003Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/vet/README

+Vet is a tool that checks correctness of Go programs. It runs a suite of tests,
+each tailored to check for a particular class of errors. Examples include incorrect
+Printf format verbs or malformed build tags.
+
+Over time many checks have been added to vet's suite, but many more have been
+rejected as not appropriate for the tool. The criteria applied when selecting which
+checks to add are:
+
+Correctness:
+
+Vet's tools are about correctness, not style. A vet check must identify real or
+potential bugs that could cause incorrect compilation or execution. A check that
+only identifies stylistic points or alternative correct approaches to a situation
+is not acceptable.
+
+Frequency:
+
+Vet is run every day by many programmers, often as part of every compilation or
+submission. The cost in execution time is considerable, especially in aggregate,
+so checks must be likely enough to find real problems that they are worth the
+overhead of the added check. A new check that finds only a handful of problems
+across all existing programs, even if the problem is significant, is not worth
+adding to the suite everyone runs daily.
+
+Precision:
+
+Most of vet's checks are heuristic and can generate both false positives (flagging
+correct programs) and false negatives (not flagging incorrect ones). The rate of
+both these failures must be very small. A check that is too noisy will be ignored
+by the programmer overwhelmed by the output; a check that misses too many of the
+cases it's looking for will give a false sense of security. Neither is acceptable.
+A vet check must be accurate enough that everything it reports is worth examining,
+and complete enough to encourage real confidence.