Add a README.md to the parent project.

README.md

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+Welcome to Public Transport Enabler, a Java library allowing you to get data from public transport providers.
+
+This project contains several subprojects:
+
+ * [__enabler__](enabler):
+     The library itself. This is probably what you're searching for. See the project's [README](enabler/README.md) for more information.
+ * [__service__](service):
+     An example of how the library could be used as a web service. It's still very incomplete though.
+
+You can build all sub-projects at once using Gradle:
+
+`gradle clean build`