dashboard: add Help button to perf pages

As per Russ' request I've described perf dashboard here:
https://code.google.com/p/go-wiki/wiki/PerfDashboard
And adding links to that page.

LGTM=bradfitz
R=adg, rsc, bradfitz
CC=golang-codereviews
https://golang.org/cl/153620045

dashboard/app/build/perf_changes.html

@@ -77,6 +77,7 @@
         <a {{if .HasPrev}}href="?page={{.Prev}}"{{else}}class="inactive"{{end}}>newer</a>
         <a {{if .Next}}href="?page={{.Next}}"{{else}}class="inactive"{{end}}>older</a>
         <a {{if .HasPrev}}href="?"{{else}}class="inactive"{{end}}>latest</a>
+        <a href="https://code.google.com/p/go-wiki/wiki/PerfDashboard">Help</a>
       </nav>
     </div>
     {{end}}

dashboard/app/build/perf_detail.html

@@ -48,6 +48,7 @@
               {{end}}
             </select>
             <input type="submit" value="Refresh" />
+            <a href="https://code.google.com/p/go-wiki/wiki/PerfDashboard">Help</a>
           </div>
         </form>
       </div>

dashboard/app/build/perf_graph.html

@@ -111,6 +111,7 @@
         </div>
 
 	<input class="button" type="submit" value="Refresh" name="refresh"/>
+        <a href="https://code.google.com/p/go-wiki/wiki/PerfDashboard">Help</a>
       </form>
     </aside>
     <div class="clear"></div>