chore(docs): update README with cli usage (#17)

README.md

@@ -15,16 +15,29 @@
 </div>
 
 ## 🔎 Overview
-Automatically generate new files with header docstring based on LICENSE information.
+Automatically generate new files or update existing codebases with header docstrings based on LICENSE information.
 
 ## 📦 Installation
-...
 ```
 cargo install docstring-rs
 ```
 
-## 🚀 Example usage
-...
+## 🚀 Usage
+```
+Usage: docstring-rs.exe [OPTIONS] --directory <DIRECTORY>
+
+Options:
+  -d, --directory <DIRECTORY>  Name of the directory in which to create new file or update existing files,
+                               if it does not already exist, creates the directory
+  -f, --file <FILE_NAME>       Name of a new file to create with docstring as header. Prepends to top of
+                               file if the target file already exists [default: *.*]
+  -l, --license <LICENSE>      Relative path to the LICENSE file to use as header docstring. If not specified,
+                               expects a LICENSE file to exist in the current working directory [default: LICENSE]
+  -u, --update                 Specify whether or not to try and update all available docstrings in a directory
+                               recursively, requires <DIRECTORY> to have been set
+  -h, --help                   Print help
+  -V, --version                Print version
+```
 
 ## 📋 License
 All code is to be held under a general MIT license, please see [LICENSE](https://github.com/wilhelmagren/docstring-rs/blob/main/LICENSE) for specific information.