add install and usage

README.md

 # Requirement Compare
 
 A CLI that generate a version changelog based on git history and local requirement file.
+
+## Installation
+
+```bash
+pip install --upgrade pip
+pip install requirement-compare
+```
+
+## Usage
+
+```bash
+$ requirement-compare requirements.txt
+- Added `last_package` version `1.0.0`
+- Bump `example_package` from `1.0.0` to `2.0.0`
+- Removed `another_package` version `1.0.0`
+```