Add README with updated instructions how to run the testsuite

Author: andreasabel

Diff for: Makefile-test

@@ -0,0 +1,41 @@
+PLT Lab 3 Test Suite
+====================
+
+Programming Language Technology (PLT, Chalmers DAT151, University of Gothenburg DIT231)
+
+This is the test suite for PLT lab 3: Code generation for C--.
+
+Prerequisites
+-------------
+
+The following tools need to be in the `PATH`:
+
+- [Haskell Stack](https://docs.haskellstack.org/en/stable/) in a recent enough version, e.g. version 3.1.1.
+- The [make](https://en.wikipedia.org/wiki/Make_(software)) tool.
+- The [java](https://en.wikipedia.org/wiki/Java_(programming_language)) virtual machine.
+
+Your solution directory needs to contain a `Makefile` with instructions
+so that the invocation of `make` there builds your solution
+and places it as executable `lab3` there.
+
+Running the testsuite
+---------------------
+
+Invoke the test runner with the path to the directory containing your solution.
+```
+stack run -- path/to/solution/directory
+```
+
+This will first call `make` in this directory and then invoke the generated `lab3` there on all the test files.
+It expects `lab3 path/to/file.cc` to generate a Java class file in the same directory, i.e. `path/to/file.class`.
+It will run the generated class files and check whether their output is correct.
+
+Getting the testsuite
+---------------------
+
+This testsuite is originally hosted at https://github.com/teach-plt/lab3-testsuite .
+It contains https://github.com/teach-plt/lab2-testsuite as a git submodule,
+so it needs to cloned recursively, e.g.:
+```
+git clone --recursive https://github.com/teach-plt/lab3-testsuite
+```