Add heroku unstability note to readme

Author: jperelli

README.md

@@ -2,12 +2,12 @@
 
 This pretends to be inspired in Google(c) static map service. But made using open source technologies.
 
-You can find osm-static-maps [running in heroku](http://osm-static-maps.herokuapp.com/ "Fabulous!")
-
 Here you have a [demo](http://osm-static-maps.herokuapp.com/?geojson=[{"type":"Feature","properties":{"party":"Republican"},"geometry":{"type":"Polygon","coordinates":[[[-104.05,48.99],[-97.22,48.98],[-96.58,45.94],[-104.03,45.94],[-104.05,48.99]]]}},{"type":"Feature","properties":{"party":"Democrat"},"geometry":{"type":"Polygon","coordinates":[[[-109.05,41.00],[-102.06,40.99],[-102.03,36.99],[-109.04,36.99],[-109.05,41.00]]]}}]&height=300&width=300 "Just what I wanted!")
 
 And a [dynamic version](http://osm-static-maps.herokuapp.com/dynamic?geojson=[{"type":"Feature","properties":{"party":"Republican"},"geometry":{"type":"Polygon","coordinates":[[[-104.05,48.99],[-97.22,48.98],[-96.58,45.94],[-104.03,45.94],[-104.05,48.99]]]}},{"type":"Feature","properties":{"party":"Democrat"},"geometry":{"type":"Polygon","coordinates":[[[-109.05,41.00],[-102.06,40.99],[-102.03,36.99],[-109.04,36.99],[-109.05,41.00]]]}}]&height=300&width=300 "Wow it gets even better!!") of the demo, for testing purposes.
 
+(heroku demo might be unstable because of too many requests being made to it, a real server costs money and we are not collecting any for this project yet)
+
 As a first approach, the service can render a geoJSON in a map, returning a PNG and you can determine also an optional height and width in pixels.
 
 Parameters that can be used (can be passed to the app server as GET query params, or POST json body (remember to set the header `Content-Type: application/json`))