Add doc for converting a TextMate theme

docs/converting-a-text-mate-theme.md

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+## Converting a TextMate Theme
+
+This guide will show you how to convert a [TextMate][TextMate] theme to an Atom
+theme.
+
+### Differences
+
+TextMate themes use [plist][plist] files while Atom themes use [CSS][CSS] or
+[LESS][LESS] to style the UI and syntax in the editor.
+
+The utility that converts the theme first parses the theme's plist file and
+then creates comparable CSS rules and properties that will style Atom similarly.
+
+### Install apm
+
+The `apm` command line utility that ships with Atom supports converting
+a TextMate theme to an Atom theme.
+
+Check that you have `apm` installed by running the following command in your
+terminal:
+
+```
+apm -h
+```
+
+You should see a message print out with all the possible `apm` commands.
+
+If you do not, launch Atom and run the _Atom > Install Shell Commmands_ menu
+to install the `apm` and `atom` commands.
+
+### Convert the Theme
+
+Download the theme you wish to convert, you can browse existing TextMate themes
+[here][TextMateThemes].
+
+Now, let's say you've downloaded the theme to `~/Downloads/MyTheme.tmTheme`,
+you can convert the theme with the following command:
+
+```sh
+apm init --theme ~/.atom/packages/my-theme --convert ~/Downloads/MyTheme.tmTheme
+```
+
+You can browse to `~/.atom/packages/my-theme` to see the generated theme.
+
+### Activate the Theme
+
+Now that your theme is installed to `~/.atom/packages` you can enable it
+by launching Atom and selecting the _Atom > Preferences..._ menu.
+
+Select the _Themes_ link on the left side and choose _My Theme_ from the
+__Syntax Theme__ dropdown menu to enable your new theme.
+
+:tada: Your theme is now enabled, open an editor to see it in action!
+
+[CSS]: http://en.wikipedia.org/wiki/Cascading_Style_Sheets
+[LESS]: http://lesscss.org
+[plist]: http://en.wikipedia.org/wiki/Property_list
+[TextMate]: http://macromates.com
+[TextMateThemes]: http://wiki.macromates.com/Themes/UserSubmittedThemes