project: update TODO, README and add CONTRIBUTING

Let's make the lives of newcomers easier :)

TODO.md

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+# Tasks
+
+ [ ] Rework internal GcalWindow working
+ [ ] Add disable/enable of views
+ [ ] Rework GcalViews as grid only (WeekView, MonthView, YearView)
+ [ ] Rework GcalViews to implement the new API (remove every "New API" comment)
+ [ ] Make GcalTimeSelector an entry
+ [ ] Properly fix timezone handling
+ [ ] Rethink signals from GcalManager
+ [ ] Order events chronologically in views
+ [ ] Change ordering of events in all-day views (use a custom ordering)
+ [ ] Add different triggers for hiding the sources view.
+ [ ] Check for the real need of adding timezone, and check everywhere
+ [ ] Connect all dconf options to the program
+ [ ] Check the real need of a caching GcalManager
+
+# Design tasks
+
+ [ ] How to handle timezones?
+ [ ] How should a Single Day view look like?
+ [ ] Need mockups for:
+   [ ] Day view
+   [ ] Agenda view
+   [ ] Timezone selector
+ [ ] What to do when there's no host set in the event.
+
+# Wishlist
+
+ [ ] Make GcalEventWidget a composite widget
+   [ ] Turn into a GtkGrid subclass
+   [ ] Remove all the custom drawing madness
+ [ ] Add resize-by-handlers capabilities to GcalEventWidget