doc/README.md: note simpler symbol links

doc/README.md: note simpler symbol links

Document that links to symbols in the standard library can be
written as "[foo]", without the actual link.

For #64169.

Change-Id: I9d8a33e85df70037320a169d55a2bb4a8a981ebf
Reviewed-on: https://go-review.googlesource.com/c/go/+/577915
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>