Updated Developer Guide (markdown)

Developer-Guide.md

@@ -99,10 +99,18 @@ To run the `check` target use:
 
     $ ./build.py check
 
+If you get this error
+
+    ReferenceError: Can't find variable: goog
+
+      file:///home/elemoine/public_html/openlayers/ol3/test/ol.html:67
+    Failed to start mocha: Init timeout
+
+it means that you don't have the Plovr server running.
+
 If you want to run the full suite of integration tests, see "Running the integration
 tests" below.
 
-
 ## Running examples
 
 To run the examples  you first need to start the Plovr web server:
@@ -156,6 +164,15 @@ first from http://phantomjs.org/download.html).
 
 See also the test-specific [README](test/README.md).
 
+If you get this error
+
+    ReferenceError: Can't find variable: goog
+
+      file:///home/elemoine/public_html/openlayers/ol3/test/ol.html:67
+    Failed to start mocha: Init timeout
+
+it means that you don't have the Plovr server running.
+
 ## Running the integration tests
 
 When you submit a pull request the [Travis continuous integration