Merge pull request #998 from OlegHahm/test_float_readme

tests: added README.md for test_float

tests/test_float/README.md

+Expected result
+===============
+This application should infinitely print '-' characters. If it prints only a single '+' characters the test must be considered as failed.
+
+Background
+==========
+This test was introduced due to an error for floating point handling in an older newlib version.
+
+The idea for this test is taken from:
+http://sourceware.org/ml/newlib/2010/msg00149.html