Update testing docs

Author: trnubo

README.md

@@ -39,6 +39,19 @@ Or alternatively specify the environment variables:
 
 Some subcommands require additional environment parameters.
 
+## Testing
+
+[bats](https://bats-core.readthedocs.io/en/stable/index.html) is used for testing. To test the image and commands bats and docker are required. Use the following commands to run all of the tests.
+
+```
+make build-with-cache # or make build
+make test
+```
+
+All tests are kept in `tests/` and all of the extension `.bats`. `test_functions.bash` is also loaded by each test. The functions include a setup and teardown (see bats docs) which creates and destroys a postgres target server.
+
+Using bats setup and teardown and avoiding exposing postgres ports etc should allow tests to be run in parallel.
+
 ## Status
 
 Feature incomplete. Work in progress.