Improve the documentation that will be installed in the mysql-test RPM.

--- mariadb-10.1.13/mysql-test/README.orig	2016-03-24 10:12:10.000000000 +0100
+++ mariadb-10.1.13/mysql-test/README	2016-03-29 10:23:30.074326900 +0200
@@ -1,15 +1,28 @@
-This directory contains a test suite for the MySQL daemon. To run
-the currently existing test cases, simply execute ./mysql-test-run in
-this directory. It will fire up the newly built mysqld and test it.
-
-Note that you do not have to have to do "make install", and you could
-actually have a co-existing MySQL installation. The tests will not
-conflict with it. To run the test suite in a source directory, you
-must do make first.
-
-All tests must pass. If one or more of them fail on your system, please
-read the following manual section for instructions on how to report the
-problem:
+This directory contains a test suite for the MariaDB daemon. To run
+the currently existing test cases, execute ./mysql-test-run in
+this directory.
+
+For use in Red Hat distributions, you should run the script as user mysql,
+who is created with nologin shell however, so the best bet is something like
+       $ su -
+       # cd /usr/share/mysql-test
+       # su -s /bin/bash mysql -c "./mysql-test-run --skip-test-list=rh-skipped-tests.list"
+
+This will use the installed mysql executables, but will run a private copy
+of the server process (using data files within /usr/share/mysql-test),
+so you need not start the mysqld service beforehand.
+
+The "--skip-test-list=rh-skipped-tests.list" option excludes tests that are
+known to fail on one or more Red-Hat-supported platforms.  You can omit it
+if you want to check whether such failures occur for you.  Documentation
+about the reasons for omitting such tests can be found in the file
+rh-skipped-tests.list.
+
+To clean up afterwards, remove the created "var" subdirectory, eg
+       # su -s /bin/bash - mysql -c "rm -rf /usr/share/mysql-test/var"
+
+If one or more tests fail on your system, please read the following manual
+section for instructions on how to report the problem:
 
 https://mariadb.com/kb/en/reporting-bugs
 
@@ -26,7 +39,8 @@ other relevant options.
 
 With no test cases named on the command line, mysql-test-run falls back
 to the normal "non-extern" behavior. The reason for this is that some
-tests cannot run with an external server.
+tests cannot run with an external server (because they need to control the
+options with which the server is started).
 
 You can create your own test cases. To create a test case, create a new
 file in the t subdirectory using a text editor. The file should have a .test