README.vendor: add note about GOROOT, recommend fresh go

Using a different build of Go (specifically, a different GOROOT) to
maintain the vendor directory doesn't always reproduce the same results.
This can result in unknowingly creating a vendor directory that isn't
able to build Go.

Add a note to README.vendor to point this out. Specifically, mention
that a mismatched GOROOT is an issue, and recommend using a fresh build
of Go to maintain the vendor directory.

Updates #69235

Change-Id: Id80c7607bf28bd76e43e1fdc672811c50f2bffb3
Reviewed-on: https://go-review.googlesource.com/c/go/+/616815
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Sam Thanawalla <samthanawalla@google.com>
Reviewed-by: Sam Thanawalla <samthanawalla@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

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