Update README.md

Add link to gitter + update copyright
Update Polish translation
[po] * update po for zh_TW
Merge pull request #197 from ycktw/master

[po] * update po for zh_TW
Merge pull request #196 from psokol/master

Update Polish translation
Add Bulgarian translation
Merge pull request #199 from SvetlinZarev/bulgarian_translation

Add Bulgarian translation
Update all po files
Add GNOME Shell 3.12 as good version in metadata.json
Corrected Persian Translation File