stack-monitor: README added

stack/monitor/README.txt

+* This stack has for purpose to know if all promises, services, custom monitoring scripts went/are ok.
+
+* If you want to use the monitoring stack:
+
+1/ In the software.cfg of your Software Release, extends the stack
+2/ In the template that will be copied for the buildout in the instance folder (instance.cfg ?), you have to add these parts:
+  certificate-authority
+  ca-nginx
+  directory
+  deploy-monitor-script
+  deploy-rss-script
+  cron
+  cron-entry-monitor
+  cron-entry-rss
+  make-rss
+  nginx-service
+  slap-parameters
+
+* If you want to add a custom monitoring script, you can write it (in whatever language you wish) and save it in YOUR_INSTANCE_FOLDER/etc/monitor.
+The only thing to know, is that if your script successfully passed, do not return or print nothing. If there is a problem, you can print the explanation on stdout or stderr
+
+* Here are 2 promises that you can add to your instance buildout, to see if it is working (one is ok, not the other) :
+[google-promise]
+recipe = slapos.cookbook:check_url_available
+path = $${directory:promise}/google
+url = http://www.google.com
+dash_path = ${dash:location}/bin/dash
+curl_path = ${curl:location}/bin/curl
+
+[failing-promise]
+recipe = slapos.cookbook:check_url_available
+path = $${directory:promise}/fail
+url = http://127.0.0.2
+dash_path = ${dash:location}/bin/dash
+curl_path = ${curl:location}/bin/curl
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