update readme to reflect contribution process

[ben-wes]

Since the process of documentation contributions will slightly change after https://github.com/pure-data/pddp/issues/173, I suggest to reflect this in the readme for new contributors.

[porres]

I did it, you can aso suggest a modification

[ben-wes]

I did it, you can aso suggest a modification

looks good - but the link in the end needs to point to the branch in lowercase (I get a 404 with the current link): https://github.com/pure-data/pure-data/tree/documentation