add link to unofficial python implementation

[Twista]

Hello,

First, thanks for the library - its awesome (using JS version for some time) - Few months ago I made port to the python and thought it could be handy to mention the link in Readme. The implementation is 1:1 to your code (same tests w/ same results) and supports both, Python 2 and 3.

[Twista]

the build failed, but it does not seem to be mine fault at all :/

is it ok to ignore it (as the whole change is just update of Readme) ?

[Twista]

Yup, its there :)

what do you think ? (could be seen here https://github.com/Twista/polylabel#ports-to-other-languages)

[mourner]

Looks good, thank you!