README.vendor: document release cycle considerations

Write down a few more things relevant to the process of maintaining
the vendor directory for the std and cmd modules. This includes the
import path of the package containing a tree consistency check, and
some considerations regarding the Go release cycle.

Also mention the updatestd command with enough context so that it's
clear that it is okay to either use it to update everything at once,
or instead update specific modules with individual 'go get' calls,
whichever is a better fit for the task at hand.

For #31806.
For #36905.

Change-Id: I37d391c557d6cf6ebf2751d71ab9c8f7c10d922d
Reviewed-on: https://go-review.googlesource.com/c/go/+/739301
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Ian Alexander <jitsu@google.com>
Reviewed-by: Damien Neil <dneil@google.com>
TryBot-Bypass: Dmitri Shuralyov <dmitshur@golang.org>

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