Modernize README using Markdown

Signed-off-by: Philippe Proulx <[email protected]>

Author: eepp

README

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+ctf-testsuite
+=============
+
+The Common Trace Format (CTF) test suite is intended to validate the
+conformance of CTF readers.
+
+To run the test suite:
+
+  1. Set the `CTF_READER_BIN` environment variable to your CTF reader
+    executable, e.g.:
+
+        export CTF_READER_BIN=my-ctf-reader
+
+  2. Execute the `run.sh` script found in the `tests/x.y` directory,
+     where `x.y` is the CTF specification's version you want to check,
+     e.g.:
+
+        ( cd tests/1.8 && ./run.sh )
+
+You may also set the `CTF_READER_OPTS` environment variable to options
+to pass to the CTF reader defined by `CTF_READER_BIN`.