Update readme

Make markdown and add install notes

README.md

@@ -0,0 +1,40 @@
+ExtEdit
+=======
+
+ExtEdit is a Thunderbird/Icedove extension to allow editing an email in an
+external editor, such as Vim or Emacs.
+
+It is based on
+[Alexandre Feblot's External Editor extension](http://globs.org/articles.php?lng=en&pg=2)
+and updated by [Brian M. Carlson](https://github.com/bk2204/extedit).
+
+To build, simply type `make`.
+
+Installation
+------------
+
+Either download the XPI or make it from source.
+
+In thunderbird, go `Tools -> Add-Ons -> Extensions` and select the
+gear to install.  Restart as required.  You can update preferences
+here, such as the path to the editor.
+
+When editing an email, you will have `Tools -> Edit in External
+Editor`.  You can also right-click on the toolbar and `Customize` to
+add a button, or use the `ctrl-E` shortcut.
+
+License
+-------
+
+ExtEdit is licensed under your choice of the Mozilla Public License, version 1.1
+or the GNU General Public License, version 2 or later.
+
+Contributing
+------------
+
+Pull requests are welcome.  The standard is to use hard tabs, aligned at four
+spaces.  The current codebase is moving in that direction, even if it isn't
+there yet.
+
+If you're submitting code, please sign off your work in compliance with the
+Developer's Certificate of Origin, version 1.1.