Document the new '$' suite file

Including the $ in a directory of yaml files causes teuthology to randomly
select one of the yaml files in that directory.

Signed-off-by: Warren Usui <[email protected]>

Author: wusui

qa/README

@@ -47,6 +47,18 @@ item), so this is really 2x2 = 4 tests, which are
   a + b2 + c + d1
   a + b2 + c + d2
 
+A directory with a magic '$' file represents a test where one of the other
+items is chosen randomly. For example,
+
+suites/foo/$
+suites/foo/a.yaml
+suites/foo/b.yaml
+suites/foo/c.yaml
+
+is a single test.  It will be either a.yaml, b.yaml or c.yaml.  This can be
+used in conjunction with the '%' file in other directories to run a series of
+tests without causing an unwanted increase in the total number of jobs run.
+
 Symlinks are okay.
 
 The teuthology code can be found in https://github.com/ceph/teuthology.git