docs: Add usage & install instructions

README.md

@@ -1 +1,33 @@
 # goexpandenv
+
+Simple command line tool to expand environment variables in a file written in pure Go.
+
+## Usage
+
+```
+Usage: goexpandenv <inputfile> <outputfile>
+```
+
+See [example](https://github.com/sikalabs/goexpandenv/tree/master/example) directory for an example.
+
+## Installation
+
+#### Linux (amd64):
+
+```
+curl -fsSL https://raw.githubusercontent.com/sikalabs/goexpandenv/master/install.sh | sudo sh
+```
+
+#### Mac
+
+```
+brew install sikalabs/tap/goexpandenv
+```
+
+#### Windows
+
+Install unsing [scoop](https://scoop.sh/)
+
+```
+scoop install https://raw.githubusercontent.com/sikalabs/scoop-bucket/master/goexpandenv.json
+```