add package description

Author: andogq

Cargo.toml

@@ -7,6 +7,7 @@ authors = ["Tom Anderson <tom@ando.sh>"]
 repository = "https://github.com/andogq/assert_layout"
 license = "MIT OR Apache-2.0"
 
+description = "Assert struct layouts, including field sizes and offsets."
 keywords = ["assert", "layout", "sizeof", "offset"]
 categories = [
     "encoding",