Rewrite Travis section; include option to use this instead of local build tools

Developer-Guide.md

@@ -21,6 +21,20 @@ Contributor License Agreement](https://docs.google.com/spreadsheet/viewform?form
 You will obviously start by
 [forking](https://github.com/openlayers/ol3/fork_select) the ol3 repository.
 
+### Travis CI
+
+The Travis CI hook is enabled on the Github repository. This means every pull request 
+is run through a full test suite to ensure it compiles and passes the tests. Failing
+pull requests will not be merged.
+
+Although not mandatory, it is also recommended to set up Travis CI for your ol3 fork.
+For that go to your ol3 fork's Service Hooks page and set up the Travis hook.
+Then every time you push to your fork, the test suite will be run. This means
+errors can be caught before creating a pull request. For those making
+small or occasional contributions, this may be enough to check that your contributions
+are ok; in this case, you do not need to install the build tools on your local environment
+as described below.
+
 ### Development dependencies
 
 The minimum requirements are:
@@ -51,11 +65,6 @@ If you are want to modify the WebGL shaders, you'll need
     $ git clone https://code.google.com/p/glsl-unit/ build/glsl-unit
 
 
-### Travis CI
-
-Although not mandatory it is recommended to set up Travis CI for your ol3 fork.
-For that go to your ol3 fork's Service Hooks page and set up the Travis hook.
-
 ## Working with the build tool
 
 As an ol3 developer you will need to use the `build.py` Python script. This is