Update README.md

Update the installation instructions
Added support for URL management via file.

    I added a second method to manage the feeds using a file.
    There is a new settings field specifing a path to a file. The user can then
    add one feed url per line.
    A second new settings field is used to switch between the two methods.
    I also added a new context menu entry which reloads the file.
Changed feed file settings type to a filechooser.

    The feed file can now be selected via a filechooser.
    The applet also does not crash anymore, if the file is not
    readable, but displays an error message in the popup menu.
Settings file chooser now depends on checkbox.

    The settings file chooser is now only active if the file is
    selected as source via checkbox.
Bump version to 0.7
Submenu experiment WIP
Change FeedTitleItem class into generic feed display class

- Started splitting the menu building into class methods so it can be
  updated separately
Feed reader as a child of FeedDisplayMenuItem

- Each FeedDisplayMenuItem now completely manages a single feed
  including handling update and error callbacks and supplying the owner
  applet with summary info (title and unread_count)
- This amounts to a significant refactor of the applet code, but will
  hopefully prove more maintainable going forward
Make sure feed display params are set before creating FeedMenuDisplayItems
First implementation of "accordion" effect

- Auto-open first feed submenu
- Close other submenus when a submenu is opened

Conflicts:
	applet.js
Get FeedDisplayMenuItem width from submenu items
Merge branch 'submenus' of https://github.com/jonbrett/cinnamon-feeds-applet into submenus

Conflicts:
	applet.js
Fix accordion menu toggling
Submenu unread count may be different from reader unread count

- Since submenus may not display all unread items due to max_item limit,
  we should only count the number of unread items shown in the menu when
  displaying a tooltip etc
Fix layout in Cinnamon 2.0.8

- Applet.MenuItem now uses an internal table for layout. This throws the
  whole layout idea of the FeedDisplayItem. We now use a table inside
  this item to make layout a whole lot easier
Add feed status icon to FeedDisplayMenuItem

- Icon indicates if feed contains unread items
Remove submenu triangle
Make FeedMenuItem layout consistent in Cinnamon 1.8.x

- Layout of underlying Applet.MenuItem has changed in Cinnamon 2.0 (now
  uses a table). Since the implementation of Applet.MenuItem is trivial
  and we need to make the layout consistent I have re-implemented
  FeedMenuItem as a direct subclass of PopupBaseMenuItem instead
- This isolates use from the layout changes in 1.8.x -> 2.0.x
Merge branch 'submenus' of https://github.com/jonbrett/cinnamon-feeds-applet into submenus
Fix read_item icon update

- Needed adjustment due to recent layout changes in FeedMenuIcon.
- Now just updates icon name instead of add/remove icon, resulting in
  simpler code too