Really add a document on testing strategy

TESTING.md

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+### Notes on Pangolin Testing
+
+The pangolin repository uses [Github Actions](https://github.com/features/actions) to provide continuous
+integration testing. The files for this are in [.github/workflows].
+
+#### Testing philosophy
+
+The [.github/workflows/pangolin.yml] workflow does an install of pangolin followed by a simple run to
+test that the tool is working. Tests can be added to that file to validate the output of the pangolin command.
+This is essentially a functional test for the tool.
+
+The test currently runs on every single pull request or push. It could be optimised, for example, to only
+respond to action on some branches, or to do a check of which files have been changed before firing off the
+complete testing workflow. For example, one should not need to test the pangolin tool if the only thing that
+has changed is the README.md.
+
+Also left as work for the future is a unit testing framework to test the components that make up pangolin.