Add download metrics

Author: Ioannis Giagkiozis

README.md

@@ -16,15 +16,15 @@
     <a href="https://github.com/igiagkiozis/plotly/actions">
         <img src="https://github.com/igiagkiozis/plotly/workflows/build_master/badge.svg" alt="build status">
     </a>
-    |
     <a href="https://crates.io/crates/plotly">
         <img src="https://img.shields.io/crates/v/plotly.svg" alt="Crates.io">
     </a>
-    |
+    <a href="https://crates.io/crates/plotly">
+        <img src="https://img.shields.io/crates/d/plotly" alt="Downloads">
+    </a>
 	<a href="https://docs.rs/plotly">
         <img src="https://docs.rs/plotly/badge.svg" alt="Documentation">
     </a>
-    |
     <a href="">
         <img src="https://img.shields.io/badge/Minimum%20Rust%20Version-1.31-brightgreen.svg" alt="Minimum Version">
     </a>