Add .txt -> .md symlink

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>

diff --git a/common-trace-format-specification.txt b/common-trace-format-specification.txt
new file mode 120000 (symlink)
index 0000000..e850727
--- /dev/null
+++ b/common-trace-format-specification.txt
@@ -0,0 +1 @@
+common-trace-format-specification.md
\ No newline at end of file