Add README.md file content

Author: gilesgas

README.md

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 # simple-python-pyinstaller-app
+
+This repository is for the
+[Build a Python app with PyInstaller](https://jenkins.io/doc/tutorials/build-a-python-app-with-pyinstaller/)
+tutorial in the [Jenkins User Documentation](https://jenkins.io/doc/).
+
+The repository contains a simple Python application which is a command line tool "add2vals" that outputs the addition of two values. If at least one of the
+values is a string, "add2vals" treats both values as a string and instead
+concatenates the values. The "add2" function in the "calc" library (which
+"add2vals" imports) is accompanied by a set of unit tests. These are tested with pytest to check that this function works as expected and the results are saved
+to a JUnit XML report.
+
+The delivery of the "add2vals" tool through PyInstaller converts this tool into
+a standalone executable file for Linux, which you can download through Jenkins
+and execute at the command line on Linux machines without Python.
+
+The `jenkins` directory contains an example of the `Jenkinsfile` (i.e. Pipeline)
+you'll be creating yourself during the tutorial and the `scripts` subdirectory
+contains a shell script with commands that are executed when Jenkins processes
+the "Deliver" stage of your Pipeline.