docs(readme) add more development instructions

The different test suites and the way test/development running Kong need
a different configuration file can be confusing at first, so this adds
some more insight as to what is needed and why.

Of course, this is the current system but nothing prevents it from being
improved in the future.

README.md

@@ -277,4 +277,3 @@ limitations under the License.
 [gitter-badge]: https://img.shields.io/badge/Gitter-Join%20Chat-blue.svg
 
 [google-groups-url]: https://groups.google.com/forum/#!forum/konglayer
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