barclock: add readme with screenshots

apps.json

@@ -1316,6 +1316,7 @@
     "description": "A simple digital clock showing seconds as a bar",
     "tags": "clock",
     "type":"clock",
+    "readme": "README.md",
     "allow_emulator":true,
     "storage": [
       {"name":"barclock.app.js","url":"clock-bar.js"},

apps/barclock/README.md

@@ -0,0 +1,6 @@
+# Bar Clock
+A simple digital clock showing seconds as a horizontal bar.
+
+| 24hr style | 12hr style |
+| --- | --- |
+| ![24-hour bar clock](screenshot.png) | ![12-hour bar clock with meridian](screenshot_pm.png) |