Merge pull request #25 from segmentio/env-var-readme

Clarify env variable usage

README.md

@@ -77,6 +77,32 @@ $ go run ./example.go -m 'Hello World!'
 Hello World!
 ```
 
+Environment Variables
+---------------------
+
+By default, `conf` will look for environment variables before loading command-line configuration flags with one important caveat: environment variables are prefixed with the program name. For example, given a program named "foobar":
+
+```
+func main() {
+ config := struct {
+ Name string `conf:"name"`
+ }{
+ Name: "default",
+ }
+ conf.Load(&config)
+ fmt.Println("Hello", config.Name)
+}
+```
+
+The following will be output:
+
+```
+$ ./foobar // "Hello default"
+$ FOOBAR_NAME=world ./foobar // "Hello world"
+$ FOOBAR_NAME=world ./foobar --name neighbor // "Hello neighbor"
+$ MAIN_NAME=world go run main.go // "Hello world"
+```
+
 Advanced Usage
 --------------