CONTRIBUTING.md: mention that GitHub PRs are accepted

x/pkgsite also accepts changes through GitHub, so it has to be
mentioned in the CONTRIBUTING.md file as a valid process.

Fixes golang/go#43100

Change-Id: I6d90e7f1c14d4a1d1581c453ce9136eb7c51ae5b
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/277913
Trust: Jamal Carvalho <jamal@golang.org>
Trust: Julie Qiu <julie@golang.org>
Reviewed-by: Julie Qiu <julie@golang.org>

CONTRIBUTING.md

@@ -21,10 +21,17 @@ All issues related to x/pkgsite are labeled with the
 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%3Apkgsite+label%3A%22help+wanted%22+).
 
+If this is your first time contributing to x/pkgsite, take a look at the
+[`Suggested` issues](https://github.com/golang/go/issues?q=is%3Aissue+is%3Aopen+label%3Apkgsite+label%3A%22Suggested%22+label%3A%22help+wanted%22) for a good first issue.
+
 Before you begin working on an issue, please leave a comment that you are claiming it.
 
 ## Getting started
 
+We accept contributions through
+[Gerrit](https://golang.org/doc/contribute.html#sending_a_change_gerrit) or
+[GitHub PRs](https://golang.org/doc/contribute.html#sending_a_change_github). To get started:
+
 1. Complete the steps in the
    [Go Contribution Guide](https://golang.org/doc/contribute.html).