explicitly state env precedence and convention

Author: AJ ONeal

README.md

@@ -110,6 +110,31 @@ content := getRemoteFileContent()
 myEnv, err := godotenv.Unmarshal(content)
 ```
 
+### Precendence & Conventions
+
+Existing envs take precendence of envs that are loaded later.
+
+The [convention](https://github.com/bkeepers/dotenv#what-other-env-files-can-i-use)
+for managing multiple environments (i.e. development, test, production)
+is to create an env named `{YOURAPP}_ENV` and load envs in this order:
+
+```go
+env := os.Getenv("FOO_ENV")
+if "" == env {
+  env = "development"
+}
+
+godotenv.Load(".env." + env + ".local")
+if "test" != env {
+  godotenv.Load(".env.local")
+}
+godotenv.Load(".env." + env)
+godotenv.Load() // The Original .env
+```
+
+If you need to, you can also use `godotenv.Overload()` to defy this convention
+and overwrite existing envs instead of only supplanting them. Use with caution.
+
 ### Command Mode
 
 Assuming you've installed the command as above and you've got `$GOPATH/bin` in your `$PATH`