Update README.md refering transifex localization platform

As it just was said in https://github.com/DrKLO/Telegram/issues/300 there are still a bunch of pull requests for translation of telegram.

https://github.com/monsieurbecker: «there are so many people who "waste" their time by proposing translations (for example: #286 #288 #289 #297 #299) that already exist on Transifex.»

And : #301  #304