docs: Add crates.io badge

Author: johanhelsing

README.md

@@ -1,5 +1,6 @@
 # bevy_sparse_grid_2d
 
+[![crates.io](https://img.shields.io/crates/v/bevy_sparse_grid_2d.svg)](https://crates.io/crates/bevy_sparse_grid_2d)
 ![MIT/Apache 2.0](https://img.shields.io/badge/license-MIT%2FApache-blue.svg)
 
 An opinionated 2D sparse grid made for use with Bevy. For storing and querying entities.