Add documentation links and short message for AtCoder compatibility

proconio/README.md

@@ -5,7 +5,7 @@
 
 Easy IO library for competitive programming.
 
-`proconio` provides an easy way to read values from stdin (or other source).  The main is `input!` macro.
+`proconio` provides an easy way to read values from stdin (or other source). The main is `input!` macro.
 
 The macro's user interface is basically the same with [tanakh's input macro](https://qiita.com/tanakh/items/0ba42c7ca36cd29d0ac8).
 
@@ -23,3 +23,14 @@ println!("{} {} {}", n, m, l);
 ```
 
 For more details, see documentation.
+
+- [Documentation for latest v0.3](https://docs.rs/proconio/0.3)  
+  v0.3.x is AtCoder 2020 compatible version.
+- [Documentation for latest version](https://docs.rs/proconio)  
+  The latest version containing new features and bug fixes.
+
+## AtCoder 2020 compatibility
+
+The latest version (v0.4.x) has some incompatible new features and bug fixes
+with the version in AtCoder 2020 judge server. We strongly recommend that you
+use v0.3.x in your local environment too.