CONTRIBUTING.md: update information

* Update various links
* Add tips for code review and using Gerrit

Change-Id: I8a8a194a1a4506251f8b243a4ee015b1eff7d2d5
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/240947
Reviewed-by: Jonathan Amsterdam <jba@google.com>

CONTRIBUTING.md

@@ -4,7 +4,7 @@ Pkg.go.dev is part of the Go open source project. We would love to receive your
 contributions!
 
 Since we are actively working on the site, we ask that you
-[file an issue](https://golang.org/s/discovery-feedback) and claim it before
+[file an issue](https://golang.org/s/pkgsite-feedback) and claim it before
 starting to work on something. Otherwise, it is likely that we might already be
 working on a fix for your issue.
 
@@ -15,10 +15,11 @@ any contributions for
 
 ## Finding issues
 
-All issues are labeled with the
-[`go.dev` label](https://github.com/golang/go/issues?utf8=%E2%9C%93&q=is%3Aissue+is%3Aopen+label%3Ago.dev).
-Issues that are suitable for contributors are additionally tagged with the
-[`help wanted` label](https://github.com/golang/go/issues?utf8=%E2%9C%93&q=is%3Aissue+is%3Aopen+label%3Ago.dev+label%3A%22help+wanted%22+).
+All issues related to x/pkgsite are labeled with the
+[`go.dev` label](https://github.com/golang/go/issues/labels/go.dev).
+
+In particular, we would love your help on issues tagged with the
+[`help wanted` label](https://github.com/golang/go/issues?q=is%3Aissue+is%3Aopen+label%3Ago.dev+label%3A%22help+wanted%22+).
 
 Before you begin working on an issue, please leave a comment that you are claiming it.
 
@@ -32,6 +33,9 @@ Before you begin working on an issue, please leave a comment that you are claimi
 
 3. Review the [design document](doc/design.md).
 
+4. If you are contributing a CSS change, please review the
+   [Go CSS Coding Guidelines](https://github.com/golang/go/wiki/CSSStyleGuide).
+
 ### Running pkg.go.dev locally
 
 There are two ways to run pkg.go.dev locally.
@@ -50,6 +54,22 @@ See [doc/frontend.md](doc/frontend.md) for details.
 2. Ensure your commit message is formatted according to
    [Go conventions](http://golang.org/wiki/CommitMessage).
 
+## Tips for Code Review
+
+After addressing code review comments, mark each comment as:
+
+- "Done" if you did it exactly as described
+- "Done" with a reply if you did it, but with a variation
+- "Ack" if either it wasn't a request for action, or it wasn't necessary to
+  take the action
+- A reply to continue the discussion.
+
+If a CL is in progress, but you want to push the intermediate state, it is
+helpful to mark the CL as “work in progress”. You can do this using the
+three-dot menu at top right corner, and clicking `Mark as work in progress`.
+Please do this to indicate to the reviewer(s) that a CL isn’t ready for review.
+
 ## Questions
 
-You can find us in the #tools channel on the Gophers Slack.
+If you are interested in contributing and have questions, come talk to us in the
+#pkgsite channel on the [Gophers Slack](https://invite.slack.golangbridge.org)!