pw_watch: Watch for Markdown (*.md) file changes

Change-Id: I73f7c4bbc88460806ccd18076538ea64b8e8beda
diff --git a/pw_watch/py/pw_watch/watch.py b/pw_watch/py/pw_watch/watch.py
index efda98d..1db9261 100755
--- a/pw_watch/py/pw_watch/watch.py
+++ b/pw_watch/py/pw_watch/watch.py
@@ -267,6 +267,7 @@
     '*.h',
     '*.hpp',
     '*.ld',
+    '*.md',
     '*.proto',
     '*.py',
     '*.rst',