update documentation

README.rst

@@ -16,11 +16,13 @@ and changes:
 -  support for multiple (parallel) instrument tracks
 -  a per-file header with work metadata (optional)
 -  lines starting with a ``#`` are comments
--  the alias ``~`` is not supported
+-  the alias ``~`` (itself a BSD extension) is not supported
 
 The library currently contains functions to:
 
 -  parse an (extended) MML file into metadata and individual tracks
+-  return a normalised form of the MML that does not make use of the
+   extensions and should PLAY on the original BASIC interpreter
 -  a duration estimate and the number of tracks are added to the metadata
 -  return a list of tuples (frequency, duration) to play (in a threaded
    interpreter, such as Floppi-Music)
@@ -40,9 +42,13 @@ The library currently contains functions to:
 
 -  export (currently only the first track of) MML to an NXC program
 -  check tracks for synchronisation error
--  missing tracks, i.e. with 0 bars
--  missing bars (can only be detected at the end, of course)
--  missing notes within a bar, relative to other tracks
+
+   -  missing tracks, i.e. with 0 bars
+   -  missing bars (can only be detected at the end, of course)
+   -  missing notes within a bar, relative to other tracks
+
+- play a playlist (parsed MML) using PWM on GPIO pins on ESP boards
+  under MicroPython (experimental; may not work for higher pitches)
 
 Examples
 --------