add readme for greet module

Author: woodgear

Diff for: greeter/README.md

@@ -0,0 +1,11 @@
+# how to install
+```bash
+# build and install module into zsh module_path
+make TARGET_FOLDER=$module_path
+# unload first to use the latest module
+if zmodload | grep zext; then zmodload -u zext; fi
+# load it
+zmodload rgreeter 
+# run the new built-in greet command
+greet
+```