prepare for 0.2.1

Cargo.toml

 [package]
 name = "contracts"
-version = "0.2.0"
+version = "0.2.1"
 authors = ["karroffel <therzog@mail.de>"]
 edition = "2018"
 repository = "https://gitlab.com/karroffel/contracts"
 license = "MPL-2.0"
 readme = "README.md"
 categories = [
+    "development-tools",
+
+]
+keywords = [
     "design-by-contract",
     "contract-programming",
     "precondition",

README.md

@@ -71,7 +71,7 @@ To install the latest version, add `contracts` to the dependency section of the
 
 ```
 [dependencies]
-contracts = "0.2.0"
+contracts = "0.2.1"
 ```
 
 To then bring all procedural macros into scope, you can add `use contracts::*;` in all files you plan