Add README to the crate

Author: maciejhirsz

Cargo.toml

@@ -1,11 +1,12 @@
 [package]
 name = "toolshed"
-version = "0.6.2"
+version = "0.6.3"
 authors = ["maciejhirsz <[email protected]>"]
 license = "MIT/Apache-2.0"
 description = "Arena allocator and a handful of useful data structures"
 repository = "https://github.com/ratel-rust/toolshed"
 documentation = "https://docs.rs/toolshed/"
+readme = "README.md"
 
 [dependencies]
 fxhash = "0.2"