Add Transifex link to Contributing.md or Readme

[aha999]

There is no link, where we can see where to translate over a platform (i see you're using transifex), so please add info about in into Readme or Contributing.

[brarcher]

This app is currently not being actively developed:

https://github.com/brarcher/video-transcoder#note-from-developer

The Transifex link was removed, as no new translations are being requested as this time:

a17395b#diff-b335630551682c19a781afebcf4d07bf978fb1f8ac04c6bf87428ed5106870f5

If there is interest in forking the application, let me know.