Added Rationale section to README.md (fixes #403)

README.md

@@ -2,6 +2,12 @@
 
 [exa](https://the.exa.website/) is a replacement for `ls` written in Rust.
 
+## Rationale
+
+**exa**  is a modern replacement for the command-line program ls that ships with Unix and Linux operating systems, is a modern file lister with more features and better defaults. It uses colours to distinguish file types and metadata. It knows about symlinks, extended attributes, and Git. And it’s **small**, **fast**, and just one **single binary**.
+
+By deliberately making some decisions differently, exa attempts to be a more featureful, more user-friendly version of ls.
+
 ## Screenshots
 
 ![Screenshots of exa](screenshots.png)