add environment to documentation

README.md

@@ -23,6 +23,9 @@ You can see example usage in the `example` folder.
 
 # Global directives
 global = "global value";
+# Pull in the "PATH" environment variable
+path_env = $PATH;
+
 # Primary section
 primary {
   string = "primary string value";

forge.go

@@ -34,7 +34,8 @@
 //     FLOAT: ('-')? NUMBERS '.' NUMBERS
 //     STRING: '"' .* '"'
 //     REFERENCE: (IDENTIFIER)? ('.' IDENTIFIER)+
-//     VALUE: BOOL | NULL | INTEGER | FLOAT | STRING | REFERENCE
+//     ENVIRONMENT: '$' IDENTIFIER
+//     VALUE: BOOL | NULL | INTEGER | FLOAT | STRING | REFERENCE | ENVIRONMENT
 //
 //     INCLUDE: 'include ' STRING ';'
 //     DIRECTIVE: (IDENTIFIER '=' VALUE | INCLUDE) ';'
@@ -62,6 +63,10 @@
 //  * Local reference:
 //      An identifier which main contain periods which starts with a period, the references
 //      are resolved from the settings current section (e.g. .value, .sub_section.value)
+//  * Environment:
+//      An environment is a way to pull in environment variables into your config. Environment variables
+//      are identifiers which start with a dollar sign (e.g. $PATH). Environment variables are always
+//      represented as strings and are evaluated at parse time.
 //
 // Directives
 //  * Comment: