weather@mockturtl - Update 2.0.0 Yahoo API closes, API rework

Removed Yahoo Weather API support and added OpenWeatherMAP instead, with capability to extend to  support other APIs later on.

weather@mockturtl/README.md

@@ -1,24 +1,41 @@
-## cinnamon-weather
+# cinnamon-weather
 
 Adaptation of Gnome Shell's [weather extension](https://github.com/simon04/gnome-shell-extension-weather) for the [Cinnamon](http://cinnamon.linuxmint.com) desktop.
 
 cinnamon-weather uses [Semantic Versioning](http://semver.org/).  For the current version number, see `metadata.json`.  
 
 ----
+## Setup
+### OpenWeatherMap API Key Setup
+Go to [OpenWeatherMap](https://openweathermap.org/) and create an account. Select their free tier. Then go your [API keys section](https://home.openweathermap.org/api_keys) where you should find one key already created.
 
-### Requirements
+![openweathermap_api_guide_screenshot](https://user-images.githubusercontent.com/3834659/51523797-48ef4c00-1de1-11e9-9e5b-39d0116b1468.png)
+
+Copy this key and paste it into the "API Key" field in the applet configuration window.
+
+### OpenWeatherMap Location
+The following formats are supported:
+
+* City, Country Code (e.g. London, UK)
+* Zipcode, Country Code (e.g. 94111, US)
+* Latitude, Longitude (e.g. 37.77, 122.41)
+
+If the location you try does not work, try using [OpenWeatherMap's finder](https://openweathermap.org/find) and paste the coordinates.
+
+## Requirements
 
 * [Cinnamon](https://github.com/linuxmint/Cinnamon) 1.8+ 
 
 For compatibility with Cinnamon 1.7 or earlier, use an [archived](https://github.com/mockturtl/cinnamon-weather/tags) version.
 
-### Configuration
+## Configuration
 
 Right-click to access `cinnamon-settings` -> _Applets -> Configure_.
 
-### [Mailing list](http://groups.google.com/group/cinnamon-weather)
+## Mailing list
+[Mailing list](http://groups.google.com/group/cinnamon-weather)
 
-### Issues
+## Issues
 
 ###### Fedora 19 "Schrödinger's Cat"