Add a README inside tests for developer information

tests/README.md

@@ -0,0 +1,38 @@
+This directory contains tests to be run by pytest
+
+To run them, it should be possible to run pytest in the project root:
+```bash
+$ pytest -v
+```
+
+The gen_test.py file will run a few basic tests and it will also run
+generation tests defined in subdirectories with the following name patterns
+through individual tests:
+```
+test.<something>
+exception.test.<something>
+```
+
+In each of these directories, three files are expected:
+
+```
+input.yml
+template
+result
+```
+
+(Actually, in the exception case, the result is actually not needed since
+ it is supposed to fail before comparing the result)
+
+The code will read `input.yml` and generate output using `template`, and then
+compare it to `result`.
+
+Directories named `test.<something>` are required to successfully generate
+something that is identical to `result`
+
+Directories named `exception.test.<something>` are expected to throw an
+exception during parsing.  In other words a failure during parsing means a
+successful test, and otherwise the test fails. (Future change: possibly
+support exceptions during generation stage also).
+
+