doc: mention the scratch repo

If people are interested in contributing to Go, but not sure what
change they'd like to make just yet, we can point them to the scratch
repo, so they can go through the process of submitting and merging
something now, and make more useful contributions later.

My evidence that sending people to the scratch repo would encourage
future contributions is that a number of people who went through the
workshop at Gophercon have continued to send CL's after submitting to
the scratch repo, even though I doubt they planned to before going
through the workshop.

Change-Id: Ieb48415773c0ee7dc400f8bf6f57f752eca8eeb5
Reviewed-on: https://go-review.googlesource.com/49970Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

doc/contribute.html

@@ -305,6 +305,15 @@ which only bug fixes and doc updates are accepted. New contributions can be
 sent during a feature freeze but will not be accepted until the freeze thaws.
 </p>
 
+<h3 id="scratch">Not sure what change to make?</h3>
+
+<p>
+If you want to become familiar with Gerrit and the contribution process,
+but aren't sure what you'd like to contribute just yet, you can use the <a
+href="https://go.googlesource.com/scratch">scratch repository</a> to practice
+making a change.
+</p>
+
 <h2 id="making_a_change">Making a change</h2>
 
 <h3 id="checkout_go">Getting Go Source</h3>