Fix minor typo in the "Contributing" section

[shadyan]

• Changed "not" to "note" :)

Note: I wanted to add support for another language, but the current rules mechanism is a little limited when it comes to complex grammatical rules. Some things can only be (reasonably) done using regex. At one point I had about a thousand rules. Evaluating other approaches.

[florianeckerstorfer]

@shadyan Yes, me too. We decided on the simple rules approach because it is a lot faster than evaluating a list of regexp rules (like, really a lot faster). However, if you implement a custom slugifier while still implementing SlugifyInterface all the integrations still work.