doc: add developer guidelines

docs/devel_guidelines.md

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+# Developer guidelines
+
+## How to write validation tests
+
+Each validation test has a `.go` file in the `validation/` directory and can be compiled into a `.t` file and executed independently.
+
+### TAP output
+
+Each validation test prints TAP output.
+So far, we have two kinds of validation tests and they print the TAP output differently:
+* tests using `util.RuntimeInsideValidate`: they start the process `runtimetest` inside the container and `runtimetest` prints the TAP output. The test process itself must not output anything to avoid mixing its output with the TAP output. Each test can only call `util.RuntimeInsideValidate` one time because several TAP outputs cannot be concatenated.
+* tests using `util.RuntimeOutsideValidate`: they create a container but without executing `runtimetest`. The test program itself must print the TAP output.
+
+### Exit status
+
+When the runtime fails a test, the TAP output indicates so with "not ok" but the exit status of test program normally remains 0.
+A non-zero exit status indicates a problem in the test program rather than in the runtime.