CONTRIBUTING: clarify two sections

Various edits to the contribution guide, including:

- Linking to the install go-codereview section
- Adding an example of a cross repo reference

Signed-off-by: Cedric Charly <[email protected]>
Change-Id: I859f6ae7f8ffd27f09948b3bb0cb41d97b2b548c
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1171864
Unity-Result: CUE porcuepine <[email protected]>
TryBot-Result: CUEcueckoo <[email protected]>
Reviewed-by: Paul Jolly <[email protected]>
Reviewed-by: Daniel Martí <[email protected]>

Author: cedricgc

CONTRIBUTING.md

@@ -322,7 +322,7 @@ follow:
 - **Step 0**: Review the guidelines on [Good Commit Messages](#good-commit-messages), [The Review Process](#the-review-process) and [Miscellaneous Topics](#miscellaneous-topics)
 - **Step 1**: Decide which email address you want to use for contributions.
 - **Step 2**: Set up a [GerritHub](http://gerrithub.io/) account.
-- **Step 3**: Install `git-codereview`
+- **Step 3**: [Install `git-codereview`](#step-3-install-the-git-codereview-command)
 - **Step 4**: Clone the CUE repository locally.
 
 
@@ -628,10 +628,10 @@ to the change in Gerrit, but it will not close the issue when the change is
 applied.
 
 
+All issues are tracked in the main repository's issue tracker.
 If you are sending a change against a subrepository, you must use the
 fully-qualified syntax supported by GitHub to make sure the change is linked to
-the issue in the main repository, not the subrepository.  All issues are tracked
-in the main repository's issue tracker.  The correct form is "Fixes #159".
+the issue in the main repository, not the subrepository (eg. "Fixes cue-lang/cue#999").