Udate README

README.md

@@ -37,6 +37,7 @@ View the [Changelog](https://github.com/huderlem/poryscript/blob/master/CHANGELO
   * [Scope Modifiers](#scope-modifiers)
   * [Compile-Time Switches](#compile-time-switches)
   * [Optimization](#optimization)
+  * [Line Markers](#line-markers)
 - [Local Development](#local-development)
   * [Building from Source](#building-from-source)
   * [Running the tests](#running-the-tests)
@@ -60,6 +61,8 @@ Usage of poryscript:
         input poryscript file (leave empty to read from standard input)
   -l int
         set default line length in pixels for formatted text (uses font config file for default)
+  -lm
+        include line markers in output (enables more helpful error messages when compiling the ROM). (To disable, use '-lm=false') (default true)
   -o string
         output script file (leave empty to write to standard output)
   -optimize
@@ -773,6 +776,9 @@ Note, `poryswitch` can also be embedded inside inlined `mapscripts` scripts.
 ## Optimization
 By default, Poryscript produces optimized output. It attempts to minimize the number of `goto` commands and unnecessary script labels. To disable optimizations, pass the `-optimize=false` option to `poryscript`.
 
+## Line Markers
+By default, Poryscript includes [C Preprocessor line markers](https://gcc.gnu.org/onlinedocs/gcc-3.0.2/cpp_9.html) in the compiled output.  This improves error messages.  To disable line markers, specify `-lm=false` when invoking Poryscript.
+
 # Local Development
 
 These instructions will get you setup and working with Poryscript's code. You can either build the Poryscript tool from source, or simply download the latest release from the Releases tab on GitHub.