docs(mura): readme overview section (#49)

See the issue for details
- Documentations #15

README.md

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 # 🐍 Mura 
-A simple environtment variable reader for Go.
+An environment variable reader for Go that bind env value into struct variable, with zero external dependency. The main idea of ​​making this project is the use of `os` and `reflect` packages from Go.
+
+## Overview
+This package will provide following features:
+- Read system environtment variable bassed on `env` struct tag
+- Bind env value into struct 
+- Fill struct with default value if env not present