Updated README after v2.1

Updated languages and translation data.
Removed OpenWeatherMap reference, added Yahoo! Weather.
Updated Credits
Fixed typo in Spanish string

Also added a note to a string that needs to be fixed.
Update docs.

* fix typos
* break long lines
* use the relative URLs
* use code syntax in more places
Losslessly compress images.

Before: 215 KB (220.636 bytes)
After:  213 KB (218.620 bytes)