test: add read.me for tests

re6st/tests/README.MD

+If you clone this repo directly, you should add re6stnet to PYTHONPATH, so
+python can find module re6st
+
+To run all the tests, you have two methods: one is by creating a new user
+namespace, the other is by using root privileges.
+
+
+* `unshare -Unr bash -c "mount -t sysfs /sys & python -m unittest discover"`
+* `python -m unittest discover`
+
+The mount of /sys is because nemu will read files in /sys.
+When creating a new network namespace, the exiting network devices will not
+disappear, and a re-mount is needed to update /sys.
+
+If you want to only run the unit tests  `python -m unittest discover` is ok,
+the network tests will be skipped.