cmd/compile: add a 'Tips' section to README to help new contributors
This CL adds a new 'Tips' section to the cmd/compile README.
The primary intent is to help new-ish contributors.
It includes some basics on getting started, testing changes,
viewing coverage, juggling different compiler versions,
some links to additional tools, and so on.
Updates #30074
Change-Id: I393bf1137db9d2bb851f7e254b08455273ccad8c
Reviewed-on: https://go-review.googlesource.com/c/go/+/503895
TryBot-Result: Gopher Robot <gobot@golang.org> Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: t hepudds <thepudds1460@gmail.com> Reviewed-by: Keith Randall <khr@golang.org> Reviewed-by: Keith Randall <khr@google.com>