Add a CONTRIBUTING.md to clarify project structure and valid contributions (#550)

* Add a recommended project stucture
* Add description for languages with more than one unit testing framework

CONTRIBUTING.md

@@ -0,0 +1,44 @@
+Contributing to Gilded Rose Refactoring Kata
+======
+
+More translations are most welcome! I'm very open for pull requests that
+translate the starting position into additional languages. However, please
+do **not** open a pull request with your solution! It can be a bit confusing since
+GitHub encourages you to do so! Please only send me pull requests if you have a
+correction or improvement to the starting position. You don't want to spoil the
+fun of doing the exercise for other people!
+
+# Translating this code
+
+Please note a translation should ideally include:
+
+- a translation of the production code for 'update_quality' and Item
+- one failing unit test complaining that "fixme" != "foo"
+- a TextTest fixture, i.e. a command-line program that runs update_quality on the sample data for the number of days specified
+
+Please don't write too much code in the starting position or add too many unit
+tests. The idea with the one failing unit test is to tempt people to work out
+how to fix it, discover it wasn't that hard, and now they understand what this
+test is doing they realize they can improve it.
+
+If your programming language doesn't have an easy way to add a command-line
+interface, then the TextTest fixture is probably not necessary.
+
+# Recommended project structure
+
+Programming languages have a variety of conventions but the starting points try
+to maintain order among languages. Ideally, the 'update_quality' and
+Item definitions should be in a file named `gilded_rose` with your language's
+conventional casing (e.g. snake case) and location (e.g. `src/`). The "fixme" !
+= "foo" test should go in a file `gilded_rose_test` in your language's
+conventional location (e.g. `test/`). The TextTest fixture and command-line
+program, that simulates update_quality over a number of days, should go in
+`program` or `texttest_fixture`. If you can define a default for the number of
+days in the simulation please choose two days.
+
+A single sub-directory per language is not enforced. A language may have
+more than one popular unit testing framework. In that case, please add
+`{language}-{framework}/` and maintain separation between the projects. In other
+words, all the components requested should exist in both sub-directories.
+Re-using code between the directories would be confusing for those looking for a
+starting point.