tests/doc: Add intial how to test doc

This commit adds a README.md to the testing folder.
It explains the basic about how to run a test with testrunner.
It adds a reference so it will be displayed on the doxygen docs.
There currently is not obvious documentation for running tests.

doc/doxygen/riot.doxyfile

@@ -765,6 +765,7 @@ INPUT                  = ../../doc.txt \
                          src/creating-modules.md \
                          src/creating-an-application.md \
                          src/getting-started.md \
+                         ../../tests/README.md \
                          src/advanced-build-system-tricks.md \
                          src/changelog.md \
                          ../../LOSTANDFOUND.md

tests/README.md

@@ -0,0 +1,22 @@
+Running and creating tests                        {#running-and-creating-tests}
+==========================
+
+There are a number of tests included in RIOT. They are located in the
+[tests folder](https://github.com/RIOT-OS/RIOT/tree/master/tests). These tests
+allow basic functionality to be verified as well as provide an example of
+usage.
+
+
+Running automated tests
+-----------------------
+
+Some tests can be performed automatically. The test automation scripts are
+defined in the `<test_application>/tests/` folder. They are written in python
+and interact through the uart with the test application code running on a
+board to do the validation. It is recommended to flash the board with the
+test just before running it because some platforms cannot be reset while
+testing.
+
+From the test application directory run:
+
+    BOARD=<board_of_your_choice> make flash test