Add usage documentation (fix #1)

README.md

@@ -3,8 +3,39 @@
 
 Generic protocol buffer generator backed by Golang's [text/template](https://golang.org/pkg/text/template).
 
+---
+
+This is a generator plugin for the Google Protocol Buffers compiler (`protoc`).
+
+The plugin can generate files based on a template directory using the [Golang's `text/template`](https://golang.org/pkg/text/template/) engine.
+
 ## Usage
 
+`protoc-gen-gotemplate` requires a **template_dir** directory *(by default `./templates`)*.
+
+Every files ending with `.tmpl` will be processed and written in the destination folder, following the file hierarchy of the `template_dir`, and removing the `.tmpl` extension. 
+
+---
+
+```console
+$> ls -R
+input.proto     templates/doc.txt.tmpl      templates/config.json.tmpl
+$> protoc --gotemplate_out=. input.proto
+$> ls -R
+input.proto     templates/doc.txt.tmpl      templates/config.json.tmpl
+doc.txt         config.json
+```
+
+---
+
+You can specify a custom `template_dir` or enable `debug`:
+
+```console
+$> protoc --gotemplate_out=debug=true,template_dir=/path/to/template/directory:. input.proto
+```
+
+---
+
 See [examples](./examples).
 
 ## Install