add README

Author: lasconic

README.md

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+This is a plugin for MuseScore 1.2 to add harmonica tablature notation below the notes.
+It currently supports the Hohner Highlander only, for both side (A highlander and D).
+If you want to have your harmonica added to the it, please contact me, or better, do a pull request.
+
++ is for blow
+- is for draw
+
+For more info see 
+
+   - The official project page http://musescore.org/en/project/harmonica_tablature