emoji-clock: add README (#7208)

plugins/emoji-clock/README.md

+# emoji-clock
+
+The plugin displays current time as an emoji symbol with half hour accuracy.
+
+To use it, add `emoji-clock` to the plugins array of your zshrc file:
+```
+plugins=(... emoji-clock)
+```
+
+## Features
+
+| Function          | Description                                                          |
+|-------------------|----------------------------------------------------------------------|
+| `emoji-clock`     | Displays current time in clock emoji symbol with half hour accuracy  |