Add short README with basic information for contributors

See #142.

Here is my take at what I think is a minimal README for new developers. It contains pretty much what I had to find out to run the app & the tests on my computer.

There is still a bug with OS X and the usage of readlink `./lib/pkp/tools/runAllTests.sh`, I will make an other pull request for this on the concerned repository when possible.

You may have a look at it here : https://github.com/hooktstudios/ojs#open-journal-systems