Add explanation on app.py

app.py

@@ -0,0 +1,7 @@
+import streamlit as st
+
+st.success("**👈 Select a demo from the sidebar** to see some examples of what the community-based Streamlit custom components can do!")
+
+st.markdown("""
+    We picked up some custom components that work on _stlite_ from [the official featured custom component list](https://streamlit.io/components).
+""")