doc/contribute: add examples for finding issues on the issue tracker

For contributors looking for new issues to contribute to it can be
difficult to find issues that need a fix and don't already have a fix
being considered. There are several labels that help guide the way
already, like `NeedsFix`, `HelpWanted`. But many issues with this label
will already have a CL. For new contributors this can be especially
difficult.

Fixes #26494

Change-Id: Ifd38ea65e362b4c580207a06f959646e49ac594f
GitHub-Last-Rev: 6d2b54447b2ee754a6d025f5de3ebd8326e035eb
GitHub-Pull-Request: golang/go#26516
Reviewed-on: https://go-review.googlesource.com/125355


Reviewed-by: Andrew Bonventre <andybons@golang.org>
Run-TryBot: Andrew Bonventre <andybons@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>