Added link to "Using Release Channels" to the SWU settings dialog

src/octoprint/plugins/softwareupdate/templates/softwareupdate_settings.jinja2

@@ -90,6 +90,7 @@
                 <label class="control-label">{{ _('OctoPrint Release Channel') }}</label>
                 <div class="controls">
                     <select data-bind="value: config_releaseChannel, options: config_availableReleaseChannels, optionsText: 'name', optionsValue: 'key'"></select>
+                    <span class="help-inline">{{ _('Make sure you have read <a href="%(url)s" target="_blank" rel="noreferrer noopener">"Using Release Channels"</a> before switching to a release channel other than "Stable"', url="https://github.com/foosel/OctoPrint/wiki/Using-Release-Channels") }}</span>
                 </div>
             </div>
             <div class="control-group">