Update README

Author: ndthuan

.gitignore

@@ -13,6 +13,7 @@
 
 # Output of the go coverage tool, specifically when used with LiteIDE
 *.out
+*.html
 
 # Dependency directories (remove the comment below to include it)
 # vendor/

README.md

@@ -1,2 +1,20 @@
 # secretenv
-Golang package for reading secrets from file and ENV variables.
+This is a Golang package for reading secrets from files and environment variables.
+
+# Usage
+
+Inject secrets into your container.
+
+You can optionally specify an environment variable suffixed with `_FILE` to indicate the location of the secret. For example, `DB_PASSWORD_FILE=/run/secrets/db_password`.
+
+To read its content: 
+
+```go
+import "github.com/ndthuan/secretenv"
+
+func main() {
+    dbPassword := secretenv.Get("DB_PASSWORD")
+}
+```
+
+If `DB_PASSWORD_FILE` does not exist or there is an error reading it, the value of the environment variable `DB_PASSWORD` will be returned.