website: Add git remote add to contributing.md (#68)

Without adding the main gem5 repository as a remote, the command for
syncing a fork and local repository to the main gem5 repository
does not work. `upstream` needs set. This patch fixes this error.

_pages/contributing.md

@@ -302,6 +302,10 @@ This can be done via the GitHub web interface and, if so, you should `git pull`
 To do so from the command line:
 
 ```sh
+# Add the main gem5 repository as a remote on your local repository. This only
+# needs done once.
+git remote add upstream https://github.com/gem5/website.git
+
 git fetch upstream # Obtain the latest from the gem5 repo.
 git switch develop # Switch to the develop branch.
 git merge upstream/develop # Merge the latest changes into the develop branch.