doc(env): added missing docstring & README

src/env/README.md

@@ -0,0 +1,7 @@
+This module provides a framework for parsing a configuration, defined as a
+struct, from both a TOML configuration file & environment variables. Some
+notable features are:
+
+* Overwrite values in config file using environment variables
+* Allow default values in config struct
+* Read environment variable value from file

src/env/env.v

@@ -11,6 +11,11 @@ const (
 	file_suffix = '_FILE'
 )
 
+// get_env_var tries to read the contents of the given environment variable. It
+// looks for either `${env.prefix}${field_name.to_upper()}` or
+// `${env.prefix}${field_name.to_upper()}${env.file_suffix}`, returning the
+// contents of the file instead if the latter. If both or neither exist, the
+// function returns an error.
 fn get_env_var(field_name string) ?string {
 	env_var_name := '$env.prefix$field_name.to_upper()'
 	env_file_name := '$env.prefix$field_name.to_upper()$env.file_suffix'