Add documentation for generate-subunit

When the generate-subunit command was added to os-testr we neglected
to add documentation for the new command, this commit addressed that.

Change-Id: Ie643ecd2eb5faa98bfae921a9d9f96a33c7d43ba

README.rst

@@ -25,3 +25,4 @@ Features
 * ``subunit-trace``: an output filter for a subunit stream which provides
   useful information about the run
 * ``subunit2html``: generates a test results html page from a subunit stream
+* ``generate-subunit``: generate a subunit stream for a single test

doc/source/generate_subunit.rst

@@ -0,0 +1,51 @@
+.. generate_subunit:
+
+generate-subunit
+================
+
+generate-subunit is a simple tool to, as its name implies, generate a subunit
+stream. It will generate a stream with a single test result to STDOUT. The
+subunit protocol lets you concatenate multiple streams together so if you want
+to generate a stream with multiple just append the output of multiple executions
+of generate-subunit.
+
+Summary
+-------
+
+    generate-subunit timestamp secs [status] [test_id]
+
+Usage
+-----
+
+generate-subunit has 2 mandatory arguments. These are needed to specify when
+the "test" started running and how long it took. The first argument is a POSIX
+timestamp (which can returned by the date util using ``date +%s``) for when it
+started running. The second argument is the number of seconds it took for the
+execution to finish. For example::
+
+    $ generate-subunit $(date +%s) 42
+
+will generate a stream with the test_id 'devstack' successfully running for 42
+secs starting when the command was executed. This leads into the 2 optional
+arguments. The first optional argument is for specifying the status. This must
+be the 3rd argument when calling generate-subunit. Valid status options can
+be found in the `testtools documentation`_. If status is not specified it will
+default to success. For example::
+
+    $ generate-subunit $(date +%s) 42 fail
+
+will be the same as the previous example except that it marks the test as
+failing.
+
+.. _testtools documentation: http://testtools.readthedocs.io/en/latest/api.html#testtools.StreamResult.status
+
+The other optional argument is the test_id (aka test name) and is used to
+identify the "test" being run. For better or worse this defaults to *devstack*.
+(which is an artifact of why this tool was originally created) Note, this must
+be the 4th argument when calling generate-subunit. This means you also must
+specify a status if you want to set your own test_id. For example::
+
+    $ generate-subunit %(date +%s) 42 fail my_little_test
+
+will generate a subunit stream as before except instead the test will be named
+my_little_test.

doc/source/usage.rst

@@ -10,3 +10,4 @@ This section contains the documentation for each of tools packaged in os-testr
    ostestr
    subunit_trace
    subunit2html
+   generate_subunit